Why are they hiring me through a temp?

I just recently got a call about being hired on this job I've been trying to get. The lady said she wanted to hire me through a temp company for a few weeks until the senior guy there leaves in a few weeks, she said it would just be temporary and he would have to show me around the place and the ropes of it. My question is do you think that's some kinda trick or probably just wants to see that im reliable?