In the case where your pool is competing with another pool in a block-race, you want to be mining on your own block. Let’s play the scenario through with an example:
Imagine you’re a pool with 1% of the hashrate and you just found a valid block, but know that the other 99% of hashrate just switched to another block found by a competing pool.
Case A: You mine on the competing block and give up on your own block. The chance of you finding the next block are 1%. You can gain the block reward of that next block, if you find it.
Case B: You mine on your own block (e.g. by switching to it via preciousblock) and ignore the competing one. Your chance of finding the next block is still 1%, however if you do, you get the reward from your previous block and the next one.
Case B is strictly better for a pool. No matter the odds.