1. You can get access to customers you otherwise wouldn't
This is what I was talking about above. You benefit from the prospects in the other persons business, and in effect you extend your market reach. If your mailing list is 1000 strong, and you joint venture with another marketer who has a list of 1000 people, that now gives you access to 2000 people. 

2. You can leverage someone else’s knowledge or expertise
One of the other big advantages of joint ventures is that you get to benefit from someone else's expertise, and they get to benefit from yours. Let's say you are creating a new product to sell on Clickbank such as turbulance training and you are great at writing content but not so hot on writing a sales letter. Find a joint venture partner who will write you a good sales letter and then split the profits. Another example would be if you have the product to sell but aren't established in the market. You can find a joint venture partner who has ready access to the market and whom may know 'shortcuts' to make it a success faster.

3. Joint ventures mean less risk
If your business requires significant investment then splitting that investment with a partner could be a good idea. Of course you must also remember that split risk also means split profit and split control!

4. Joint ventures are great for networking
Often a joint venture means you get introduced to other influential people in your niche or market, so another advantage of joint ventures is that they are great for networking.