:help :call
:help functions
:help call()
Call/Execute a function.
Functions can be called in various ways or various contexts, and it's just like you know from any other languages.
:let MyBuf=bufname()
:echo "this is " .. bufname()
:echo strlen(bufname())
:if buflisted(1) | echo "yes, buffer 1 is listed" | endif
But when you only want to call a function (prior to Vim script 9), without assigning or passing the return value, then
you need the call command.
:call chdir('..')
Be aware that with Vim9 script the call command is no longer required