1. you can have the use of an item at a fraction of the cost of that item.
2. when you rent you don't have to worry about storage space.
There are other good reasons to rent but these two top the list.
Generally speaking an item is rented for about 1/6 to 1/8 of its cost. So renting makes good economic sense. Also, large numbers of tables and chairs take up a lot of space. With renting this problem is solved.