Storylets in Sometimes decks are not always available. There’s less chance of them being available at any one time as the deck they’re in gets bigger. Also, once they do appear, they take up a slot in the player’s hand until they’re played. So they’re hard to ignore.
The player can’t deliberately ‘grind’ (repeat and repeat playing) a Sometimes storylet. So, assuming you’ve got a decent sized deck, you can create Sometimes storylets that deliberately have generous or mean or unusual quality results. If your deck has 24 storylets, having only one that gives out a particular quality will mean that the card showing up will be a bit of a treat if the quality is useful.
Sometimes cards also provide some choice – which card to play first. It’s not a particularly significant choice though – it generally doesn’t cost anything to play a card, so the player will usually play all their cards at every draw. Sometimes cards are good for gathering resources and other repeatable content. This is because there’s no temptation to hammer at a single action over and over, which might be efficient but is very dull. Sometimes cards are poor for major goals that the character is working towards, as it’s harder to keep track of how close they are to the goal than when it’s on an Always card. Also remember that a Sometimes card effectively costs fewer actions than an Always card, as one action gets you one Always card or a handful of Sometimes cards.