You will dive more often. Owning your tanks is convenient. You are good to go whenever your buddy calls you up to dive! There is no need to wait for the shop to open in the morning to rent tanks before you head to the dive site.
They are cost-effective. Air fills for your tanks typically cost 50% to 70% less than renting them. Your tanks will have paid for themselves after 50 dives.
Tanks retain their value. The resale value of a used scuba cylinder is roughly 70% of the full retail price of buying a new one. Scuba cylinders are one of the few pieces of gear that keep their value.