Is it correct to sometimes capitalize words mid-sentence in computer user interfaces?

I've noticed that many sites and user interfaces tend to capitalize certain words or sentences like so:

Choose File

Select a File to Upload

Account Settings

How to Format

but in some other cases, they don't:

Please select a file to upload to our system

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And sometimes, particularly in list items, all in lowercase. Are these good practices? What's a reasonable rule to decide when to capitalize or not, or even when to write all in lowercase?