i know the border lines we drew between us. (cordate) wrote in rp_tutorials,
Typically, saving a file as a .psd allows you to leave your layers, effects, etc. intact in case you need to edit them later. (Saving as .jpg, .png, .gif, or any other format flattens the image and you can no longer edit your adjustments.) For example, if you took a photo and added a variety of adjustment layers, then saved the file as a .psd, you would be able to go back in and edit those adjustment layers. This is especially useful for a graphic that has text — you can just go back in and edit if you've made a typo and you won't have to start over from scratch.
For compatibility with other Adobe products, .psd files can be imported to both Illustrator and InDesign. (They're almost guaranteed to be compatible with other Adobe products as well, but I've only used them with Illustrator and InDesign.) Again, the ability to save a file with layers and as a .psd comes in handy here as well, as you can edit the .psd and you won't have to re-place the file.