Bet for value (i.e., when you think you have the best hand & you think you can get worse hands to call). Bet for bluff (i.e., when you think you can get better hands to fold). But think carefully about betting hands that may not be the best (& may not be the worst) -- that is, hands that have showdown value.
Here's some of the logic:
Let's imagine a world where you only bet for value. Your opponents are eventually going to catch on and stop calling so much. Not ideal. What's the fix? Bluffing. As noted by Hunter Cichy:
[I]f you're struggling to get value on the river, its because you aren't bluffing enough. If you bet half the post, you're offering Villain 3-to-1 on his money. He needs to be right 25% of the time to make the call. … The trick to increasing your river EV is to bluff sufficiently so that Villain has incentive to defend with most of his showdown value hands.
Advanced Concepts in No-Limit Hold'em: A Modern Approach to Poker Analysis
Why not bluff your showdown value hands? Well, those hands already have equity and so if they can make it to the river, they have a chance of winning. Middle pair can reasonably be a showdown value hand.
If you're getting +EV for your value hands and +EV on your showdown value hands (through pot control and judicious calling), where else can you extract value? The junk hands you have left. (This is not to say you should also raise, but that's where your bluffing focus should generally be.)
Here's an article on the topic:
https://newonlinepoker.com/dont-bluff-with-showdown-value/
