Why your tools must work with no signal
The places where the work happens are the places with no signal. Plant rooms. Basements. Lofts. Steel framed warehouses that behave like a Faraday cage. Half the countryside, and the inside of most things built before 1900 with walls thick enough to prove it.
Software that needs a connection in order to open is software that does not work where you are standing.
What most apps mean when they say offline
Usually this: the page loads from a cache and then sits there telling you it cannot reach the server. You can look at whatever happened to be loaded already. You cannot write anything down, which was the entire reason you got your phone out.
The distinction that decides it is where the data lives, and it predicts everything else.
If your documents live on somebody's server and the app is a window onto them, then no connection means no documents. The window still opens. There is nothing behind it. If your documents live on the device in your hand and the server holds a copy, you carry on working and the copy catches up when you drive back into coverage.
Almost every tool marketed to trades is the first kind, because the first kind is far easier to build.
What the second kind lets you do
Measure up in a basement and write the estimate down there, while the measurements are still in your head rather than on a photograph of a notepad.
Take a signature on the doorstep of a house with one bar. Print an invoice on a site with no connection at all. Look up what you charged the same customer last spring while standing in their hallway, which is both the moment you need it and the moment you are least likely to have signal.
The practical effect is that the estimate stops being an evening job. It goes out from the van before you have driven home, and the person who sent theirs on Tuesday usually beats the person who sent theirs the following Monday, regardless of price.
What to check before trusting it
Four tests, a minute each, and some well known apps fail them.
Turn the wifi and the mobile data off, then reload the page. Not navigate around it. Reload it. Plenty of apps survive being offline right up until the moment you close the tab, and then never come back.
Write something with the connection still off, close the tab completely, and reopen it. If what you typed has gone, it was never stored anywhere. It was sitting in memory waiting for a server.
Find out what happens with two devices. Write on the phone while offline, write on the laptop while online, then bring the phone back into coverage. Something has to decide which version wins, and you want to know what it decided before it matters rather than afterwards.
And find out where the only copy is. This is the test people skip.
The trade off, said plainly
Data that lives on your device is private and fast, and it is fragile in a way a server is not. Clearing your browser's site data deletes it. A wiped phone deletes it. No support desk anywhere can bring it back, because they never had it, which is the same property that made it private in the first place.
That is not an argument against local first tools. It is an argument for taking a backup you actually hold: a file on a laptop, a copy in whatever cloud drive you already pay for, taken often enough that losing the gap since the last one would be annoying rather than serious.
Signing in, where a tool offers it, buys a second copy kept by somebody else. Worth having, and not the same thing as a backup you control, because an account can be lost too.
The question to ask about any tool, online or offline, is not which of the two it is. It is this: if the phone goes in a skip tomorrow, where is the work? A tool that cannot answer that is the risk, whichever way it stores things, and plenty of cloud tools answer it no better than a notebook does.
What working locally cannot do
Being honest about the other side of it. Some things genuinely need a connection, and a tool that pretends otherwise is lying to you.
Sending an email needs one. Publishing a link for a customer to open needs one, because the customer has to be able to reach it from their house. Taking a card payment needs one. Anything that touches somebody else's system, at the moment it touches it, needs a connection.
The difference is what happens in the meantime. A well built tool lets you write the whole invoice in the basement, then sends it when you get back to the van. A badly built one will not let you write it at all, because the form that writes it lives on the server. Same lack of signal. One of them cost you the afternoon.
Paper still works, and that is the benchmark
A duplicate book has never once failed to open in a loft. No login, no update, no subscription, no server. The reason people still carry one is that it clears the bar every app is supposed to clear.
The other half of the question is what happens to the work itself, which is worth answering before rather than after: if the laptop with your business on it stopped this afternoon, what would be gone?Which is a fair standard to hold software to. If a tool cannot do in a basement what a two pound notebook does, it is not replacing the notebook, it is something you carry in addition to it. What the software is for is the arithmetic, the numbering, the copy that is still there next year, the version you can send to somebody. Those are real and they are worth having. They are worth nothing at all on the Tuesday you could not open the app.