Why do law firms ask if you have applied to them previously? Is it a disadvantage if you reapply having been rejected or...?
Because some people like to cheat the system and apply multiple times, and it helps to pick that up.
Because it helps pick up people who have applied to other roles within the firm and help work out if a TC is the right role for them.
Because a fair number of applicants will be people who applied previously, whether that is insight programmes, open days, vacation schemes or training contracts. It helps the recruiter pick up people who have come to the firm before.
Because some firms have a policy of not allowing you to reapply, or will only allow you to apply a set number of times or within a certain timeframe.
Because some firms will use it to help them judge how far your application has improved since your last application. If your application was unsuccessful last time, it helps them judge what has changed since your last application.