Do you use an actual whiteboard to practice LC?

I have one but I bought an iPad Pro and use Procreate, that's my whiteboard. I simulate an interview as closely as possible for each problem.

I found it easier to leet on a whiteboard than the website. Harder for interviewers to catch bugs on the board. But the leetcode testcases don't have any mercy on me.

These days most companies can provide laptop for coding rounds, so why not just use them

It is useful to practice on a medium where you can’t edit easily and space is limited. I used pen and paper. I avoid using laptops if possible due to the extra set-up time and possible glitches. I was given a broken laptop once.