A lot of unionized wharehouses just can't pick up a person for a few days as easy as you think. Their regular employees will feal threatened to see new people walking in off the street. Some may want to go on strike.* By going through a temporary service the new people are on the temporary companie's insurance and other plans and are easy to get rid of if they don't work well. If the new guy was suddenly in a union he could be lazy as hell and very hard to get rid of. The temporary service is a good solution for companies needing a short term employee. Just my 2 cents worth.
*A full time full time 45 year old wharhouse employee will poop his pants to see two part time 20 year olds hired on a full time basis. It's also way easier to get a part timer to work a couple of extra hours at regular pay than to get a full timer to work a couple of hours at time and a half.
Remember yesterday, respect tomorrow, live for today.
I bought some batteries, but they weren't included.