Example: reading a file line-by-line. You don’t, can’t, know how big a line is. Your best option is to allocate some random chunk of memory, like 4 kilobytes, read 4 kilobytes from the file into memory, and see if you happened across a newline in there. If you did, shuffle everything around a little / realloc / ringbuffer shenanigans; do your favourite. If you didn’t, make the buffer bigger (by 1.5x? 2x? Log2x?), and try again.

This dance is super common with variable length protocols over unframed streams - ie, most things over tcp. So this is an exceptionally common pattern in IO operations.

Other common times this pattern happens: finding all items in a list that satisfy a predicate; tracking items a user has added to an interface; cycle detection in graphs; …