It really depends what you need. Photoshop, Flash and Spriter are three completely different programs with little overlap. Photoshop is primarily for raster image design/manipulation. Flash is a development environment with a robust vector graphics and animation engine and spriter is primarily for skeletal 2d animation aimed at games.
The advantages for using something like Spriter instead of Flash or Photoshop in your game is huge though. Even though Flash has a skeletal animation system you won't be able to use it in construct, instead you will need to export each frame as a separate image and import them into construct (for smooth/complex animations this will put your VRAM usage through the roof), with spriter however you build up a character say from several small parts and then spriter itself manipulates these so the VRAM footprint is miniscule in comparison (comprising only of the few images making up a character for instance). There are other advantages too, as the spriter plugin is integrated into the Construct engine you can blend animations and the animations will also react correctly to time-scaling - allowing for some very polished effects.
Basically, if you want polished, complex animations in your construct game that aren't pre-drawn/rendered and which react correctly to the game environment Spriter is pretty much a must have.