So you test their ability to write code on the whiteboard.
I was once programming with pen and paper too (only had access to computers at university). They I bought my own computer.
I'm not a newbie (like, 15 years coding). But I haven't even bothered to properly remember standard libraries peculiarities - since they're different across the languages anyway, and with modern software stacks (that are quite diverse) it's not unusual to write Python but do some C or C++ one day (speed-ups), detour to JavaScript, try out Elm, then have a day patching some Go code that an useful tool is written in, then go learning some Elixir. Is it strpos(substring, string) or strpos(string, substring)? What does base64.encode(blob) returns, again, ASCII bytes blob or a text string? Which module/header file contains/exports that symbol? Maybe I'm doing it all wrong but I honestly never remember such details that for longer than a week or another - a proper tooling always hints those kind of things to me, so I don't even recognize I tend to forget them.
On a whiteboard, I'd have some awkward moments.
Adds some extra anxiety as interviewee still worries about their code being correct or not. No one wants to risk being frowned upon just because they've messed up some syntax fine point. And since people are different, sometimes it really happens.
This can be avoided if interviewee's specifically told they just have to write algorithms in pseudocode, but it's not how it always goes in practice.