It offers work across many sectors.
There are loads of large national agencies eg Reed, Pertemps, Hays, Manpower etc. As well as smaller local ones offering a variety of roles in a wide range of industries - everything from customer service and admin, to care work, manufacturing, warehouse work and professional industries.
What they are looking for mainly is commitment, reliability, work ethic and a reference.
My partner works in manufacturing and they take people on from agencies - a lot of their workers are agency and many have just been willing to work and turn up and have been trained on the job. A lot of them are men with no qualifications and some have little to no work history.
My brother joined an agency after university which led to a permanent graduate role.
If they're at university, they will have already demonstrated the soft, interpersonal and admin skills for a low level job. Eg group assignments shows teamwork, if you've taken notes during these meetings, you have experience of minute taking. If they're school leavers who haven't worked, any extra curricular activities etc will have given them soft skills. Agencies will find a way of spinning your skills in a positive way.
The best thing they can do is to come up with a CV and make contact with a few agencies. The consultant my daughter spoke to was really helpful.
Basically, agencies will take on anyone who applies and come across well! After that, it's down to the individual how successful they are. And agencies will pretty much tell you that.