When your deck centers around Minion as a sort of self-contained engine, typically, given a starting hand of the form (say)
Minion-Minion-Steward-Copper-Estate
you will play the first Minion for +2, then play the second for the 4 cards, hoping this process will get you to 8. In such a deck, it's better if Minions clump; Minion - Minion - Minion - Copper - Estate is almost surely a province hand; Minion - Silver -Copper - Copper -Copper is very frustrating, because it often draws another hand with exactly one Minion.
However, sometimes, the point of the deck isn't so much to make money Minion-ing as to play key action cards again and again; principally, Goons, but also Monument, possibly Knights, possibly some new cards I haven't thought about. Let's imagine a thin deck with some Minions, Goons, and Villages, but no draw other than the Minion. Then you tend to want a very different distribution of Minions in the deck In fact, assuming your deck composition is the same in both cases,
Minion-Minion-Village-Goons-Copper
can be a less desirable hand to see at the start of your turn than
Minion-Village-Goons-Copper-Copper.
Why? With hand one, if you play village, Minion for coins, Goons, and then use a Minion to draw 4 fresh cards, there's one fewer Minion for you to draw, and so the probability that you reach the second Goons in your deck is lower. But playing that second Goons is much more important than taking +2coins from the extra Minion. So clumpy Minions are bad in this deck, not good.
It gets worse if, after playing a hand like the above, you shuffle into Minion-goons-goons-copper. You could go hunting for your villages, but if you find them, will you get back to your Goons? Probably not if you've already played 3 Minions.
There is a simple, but counterintuitive, solution to this problem. Not always, but sometimes, when you draw the "clumpy-Minion" hand, you need to shuffle a perfectly good Minion back in! That is, play Village, Goons, then Minion for new cards, discarding the other Minion. The point is that later in the hand is a much better time for you to know whether you are going to need that second Minion for the draw or for the +2 coins. Committing yourself to the coins now means you can't use it for the draw later.
Again, this isn't for always; it's for thin decks where you're hopeful you'll see the Minion you discarded again. If there's little hope of that, then take the coins. This may already be obvious to experts, but it's something I've only just started doing after maybe 15000 games of Dominion, and it seems useful to share.