They tend to do just one type of law (i.e. practice area), so you need to know in advance what type of law you want to practise. The main ones paralegals practise in are:
Company & commercial
Family and child care
Road traffic accident
Paralegals also work in many, many other areas too, but these are the main areas, and thus the ones with the most vacancies.
The job of (say) a probate paralegal is quite different to (say) the job of an employment law paralegal. Employers won't take you very seriously unless you know what area you want to work in.
The person who wires the space shuttle, and the person who wires your house are both electricians, but need wildly different skills and attributes - and so it is increasingly the case with law.
Therefore applications basically saying, "I just want to be a paralegal, whatever you've got is fine." Always look lazy and unfocused compared to ones which in effect say "I want to be a paralegal doing personal injury work and you have a great reputation in that area. I would love to work for you as an entry-level paralegal and build my career as a professional PI paralegal with you."
Do your homework before applying.