Was 2021 the year where Covid significantly affected a level exams & results, and what are your current A level grades?
I Don't think any employer in law would expect multiple A*s or similar on resits, but for employers that do filter by a-levels they might expect a slightly better performance given that you're a couple of years older than the typical a level student.
It's really hard to say how employers as a whole would consider this, some strictly filter with a-levels to thin out the numbers, some don't. For those that filter it's important to hit the requirements to make the cut. For those that don't filter if the rest of your application is impressive it matters less, although law firms tend to recruit candidates with strong academic track records.
As for whether re-sitting it worth it... it really has to be a personal choice. Assuming you're at uni at the moment then re-sitting takes away time you could be spending on your degree and time you could be spending seeking out work experience, vacation scheme applications, open days etc... + the cost.