Let's examine what's going on behind the scenes so we can understand why buying an option can offset a short option of the same type. The basic idea is essentially this:
When you have a short option, you are agreeing to buy or sell 100 shares from someone else.
When you pair your short with a long option, you are reserving the right to close those shares at a predetermined price.
Since entry and exit pricing is predetermined by strike selection, you are essentially saying, "I'm willing to trade shares for one price, but I reserve the right to close them at my preferred price, no matter what happens." In other words, verticals give you a built-in exit strategy that fits your risk tolerance, and can't be denied by the market.
Verticals are highly versatile strategies. Depending on your strike selection, they can be sold for credits, or cost you debits. They can be low probability, or high probability. They can be bullish or bearish. They can be done with puts or calls. They can even be combined with each other to create more complex strategies.
With the exception of a few verticals are used to build nearly all other spreads. Learning the dynamics of basic verticals will greatly help you understand any other strategy, and give you the ability to visualize risk in your own head. You'll be able to create your own setups that fit your market assumptions, and manage your positions with speed and confidence.