In far more cases than not it will help your application. It might help less with VS applications, especially if you are not eligible for the programmes (where you have graduated) or if there can be concerns of conflict of interests checks (in many cases you maot be able to work for a competitor if you are employed by another).
In days of old there used to be an attitude in some firms that many who did paralegal work wouldn’t have the skills/attributes to go on to get a TC. There was a bit of an attitude of “why haven’t got a TC by now”. But I think that attitude has diminished a lot, especially given firms attitudes to diversity and also their want of finding people who are resilient enough to work in their demanding environment. Finding someone who has seen a similar inside workings of a firm can be a huge benefit, especially when you see the realities of the firm rather than a vacation scheme when often the true environment isn’t really shown.
There may still be some firms who are a bit sniffy about those who have done paralegal work. I would expect them to be in a small minority though these days.
It was a while back now so it may no longer be the case, but I am pretty sure
Hogan Lovells (might have been HSF) had a policy of not allowing their own paralegals apply for TCs. Other firms might have internal policies about paralegals applying for TCs. At one firm I worked at, paralegals had to have at least six months experience (and a performance review) and an endorsement from a partner, otherwise they were just treated like an external applicant.