Loan is a much-maligned card. In the mid- to late-game, Loans do more harm than good, bypassing your good Actions only to discard your Platinums or Golds. Effective use of the Loan is very situational.
First, Loan is a good defensive card. Loans can clear out the Coppers that Mountebanks add to your deck. Attacks that give you Curses are blunted when you can use the Loan to skip over worthless cards. If you plan to gain money through Actions to defend from Pirate Ships, then Loans can also quickly clear your deck of Pirate Ship targets. And if you are being attacked by Ghost Ship, then you can dump useless cards onto your deck in anticipation of using Loan to discard it.
Second, Loan is useful when there are ways to turn it into something else, e.g., Remodel, Bishop, Salvager, or Apprentice. Those cards are generally too inefficient at deck-thinning because trashing Coppers provides little to no benefit for them. Here, Loan can clear out the Coppers quickly before getting out of the way and turning into a useful $5 Action, giving 1 VP token, adding $3, or drawing 3 cards. Mines can also turn unwanted Loans into Gold, though Loan sort of anti-synergizes with a Mined deck.
Third, as alluded to earlier, Loan is helpful in godawful decks burdened with useless cards, where having to discard a Treasure is well worth being able to skip all the Curses and Victory Cards before it. Of course, this can backfire; in a deck with only one Gold, it can really sting to have to discard it. But in, say, a Goons deck, where you are buying up Coppers for VP tokens, Loan can help get rid of the junk. Alternatively, Loan is quite useful in no-treasure strategies like Minion.
Fourth, if Venture is the only good $5, and you anticipate buying several of them, Loans can be indispensable in clearing out your Coppers. The problem is that you might trigger the Loan with the Venture … which then forces you to discard another Venture.
EDIT: As noted below in the comments, this isn’t totally accurate; Loan is good in bad Treasure decks, not necessarily in bad Action decks
The real problem with Loan is that it’s only a card to buy if no viable alternatives are available. In every respect it is out-classed by something else: Reaction cards are better at dealing with attacks; Chapel is better for deck-cleansing; Ventures are a better option for getting through the deck; Watchtowers are better for Goons. And unless Loans are the only source of deck-thinning in the game, even the simple Silver is probably preferable.
The one aspect that Loan uniquely excels at is the no-Treasure deck. This might be because you are paranoid about Pirate Ships, but more likely it is because you are running Loan/Minion.
- Remodel, Salvager, Apprentice, Bishop
- Attack cards (as a defense)
- Venture (only if there aren’t other trashing options)