Some capture devices don't have an acquisition buffer so it's not the classic "trigger / capture / download" cycle, but it's just start acquisition then you get samples until you said you had enough.

With sample compression this can also be quite high rate / over long periods (so raw uncompressed samples could be very memory hungry). Like for instance if you monitor a serial bus that's idle most of the time, and you have high oversampling just to get more time resolution.