You wouldn't. The Photography plan is an excellent bargain, and I am sure you know it comes with Lightroom, Bridge, Media Encoder, Adobe Fonts, and other stuff I am sure. Check out the 'what's new' videos on the recent update. It works like magic now.
In addition, Adobe offers a single app plan for all apps which can be annual or month-by-month. Most plans are annual, as is the Photography plan which is heavily discounted. Annual plans (paid monthly or in a lump sum) have a fee for early cancellation.
The reason you might buy the Photoshop plan instead of the Photography plan is that you only need it for one or two months and want to cancel without penalty.