A debit spread buys one strike and sells a further one in the same direction — a call spread for upside, a put spread for downside. The short leg brings in premium that offsets part of what you pay, which changes the trade's character.

What the short leg buys you

Selling the further strike does three things: it lowers your cost (you pay a net debit, not full premium), it cuts theta (the short option decays in your favor, partly offsetting the long's time bleed), and it reduces vega. On a slower grind where decay is your enemy, that's a real advantage over a single long option.

What it costs you

The trade-off is a capped payoff: your gains stop at the short strike, no matter how far SPY runs. You've traded away the explosive upside of a single option for cheaper, steadier exposure. You also add a short leg (minor assignment consideration) and accept combo fills that can be slower on thin 0DTE strikes.

A debit spread trades the home run for a cheaper, lower-decay single. Great when you have a defined target — wrong when you need speed and open upside.

When it fits

A debit spread makes sense when you have a defined target — say a grind up to the call wall — and want to reach it cheaply with less decay, rather than betting on an open-ended run. Set the short strike at or near your target level so the cap costs you nothing you'd have captured anyway. For a fast in-and-out scalp, though, a single option is usually still the better tool.