[dspillett]

> Is a spreadsheet even necessary on the go?

Necessary, no, not by any means. But it is certainly a very-nice-to-have. Something far simpler than Excel would do but as it runs on my phone and I have a licence, why not?¹

> just have a calculator and store values in a text file?

That would be something new to learn and/or a manual process.

I use spreadsheets for little repetitive calcs, or even non-repetitive ones that I want to remember after I've lost the bit of paper from my pocket that they'd otherwise be on. This doesn't happen on-the-go often, but when it does it is handy to have something that lets me go back and edit the calcs easily, and automatically keeps the results rather than me copying them from calculator to notepad. The middle ground that is a logging calculator app could deal with some of these use cases, of course, as long as it has the range of functions a reasonable spreadsheet has (usually I'm just totting up money, but sometimes time-base calculations are useful, or unit conversions (though 1.609 sticks in my mind better than I remember what Excel's CONVERT() function calls miles so usually those are done from memory not with a built-in function), and so on).

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[1] other than the fact it seems to have sync problems, so I never trust it to edit workbooks created elsewhere