1. Easier to learn the niche.
2. Can learn how to get traffic
3. and how to promote your site to get targeted visitors and sales.
4. Not that depressed if it fails (did not sit and make 100 templates first)
5. Can get a site up quick
6. Can establish return traffic with high quality templates
7. Huge inventory
8. Easy Setup
9. Ability to get subaffiliates under you
10. Easier to establish trust in your company when you got quality products that look good.

If you are no good at running a template site, all you are out is a couple hours and a couple of bucks for the hosting. Creating your own template site which you have spent hours upon hours building the templates can lead to a pretty big upset if it fails.

11. Once you get a TM store set up, you can easily divert traffic to your template store.

Do you want me to go on? ;)