Hi!
This question is likely included to give candidates a fair chance to explain anything that might need context (e.g. illness, caring responsibilities, disruption to studies, resits, gaps, etc.). If you don't have any mitigating circumstances or genuinely relevant contextual information, it's absolutely fine (and often better) to leave it blank. The graduate recruitment team are not going to penalise you for this, and are likely expecting it for the majority of applicants.
I would avoid using the space to:
1. Repeat work experience or extracurriculars already covered elsewhere in your application
2. Squeeze in a "why I'd be a good trainee" answer
3. Answer "why law or why AG" directly
If the firm wanted this to be included in the application form, they likely would've made it a dedicated application question - it could be the case that these questions will come up later on in the process!
If there is a substantial extracurricular or responsibility that adds important context to your application and didn't fit elsewhere in your application, I'd say that you can add this very briefly if you feel it would add value to your application. If not, it's totally fine to leave it blank