For some this may make no sense as to why one would want something to eat their fish, if they are trying to grow and raise them. I felt exactly the same way at first.
When you purchase your tilapia, the majority, and I am talking about 98-99% here will be male. The reason for this is they are given a hormone at the start of their life which can alter their sex. 1-2% doesn't seem like a lot of females but when you realize the amount of eggs a female can produce, and many of those will be females. you can see how the problem can escalate.
There are several reasons why you don't want breeding to occur. The first is you will have a population explosion. These fish will all be competing for food. As such none will grow to the intended size you had hoped for.
The second problem will be as the fish are breeding, they make holes (nests) along the edge and bottom. This will result in an uneven bottom making netting much more difficult. When the time comes to net the lake, the fish will be able to escape below the net using one of these holes.
Another problem which we were unaware of at first was the behavior of the fish. We netted some larger fish and put them in our tanks waiting for them to sell the following weekend. It was here where we saw behavior which would also be occurring in the lake, although unseen.
In tanks with a female, she would stay in a corner guarded by a male. The other fish would hover around waiting for their chance to be 'top male'. They were so interested in her, they weren't eating. Tilapia which aren't eating aren't gaining weight and aren't going to earn you money.
It is for the above reasons why you need predators if you have your fish swimming freely and don't sell all of them before breeding begins, which can be as early as 4 months