Since the latest gym update in Pokémon GO, you are encouraged (by repeated messages) to feed berries to your Pokémon. However, since the amount of Pokécoins is capped at 50, equalling about 8 hours, why would you want to do that?
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Every Pokémon will stay in the gym until it is kicked out. Berry or no berry.
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If a gym is attacked, it is usually lost. I am only a level 30 player, but can clear out any gym alone. Berries might delay that if fed during the attack, but in practice I've never seen that happen.
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After 8 hours, you want your Pokémon to return to you, not stay.
So, why would I feed a berry?
Best Answer
There are a few reasons you would want to feed a defending Pokémon a berry.
Feeding a Pokémon in a gym a berry earns you a small amount of experience towards that gym badge. Having higher level gym badges means more rewards when spinning that gym's disc. It also frees up inventory space by getting rid of your Nanab berries you likely aren't using. Furthermore, there is a small chance to obtain a single candy for the family of Pokémon that you fed a berry to.
Additionally, if the gym is under attack (and your Pokémon hasn't been there for 8 hours yet), you could feed your Pokémon berries to top it off while the attacker(s) is/are attacking, which could easily demoralize them and cause them to give up when they see that the defending Pokémon's owners are active and nearby.