I personally would describe an encoder as your coding books on steroids. Besides everything in your CPT/ICD/HCPCS, you will have additional descriptions of each code. It will give you RVUs, NCCI edits, LCDs/NCDs, global periods, allowed modifiers, MUE, Medicare fee schedule. It's all the info in your books, plus just a whole lot more. You can make favorites, and notes (that don't have to be recopied into new paper books every year). It's not a magic wand where you type in a single word, and it gives you the exact CPT/ICD10 you should use. You still need to read the descriptions, coding guidelines, etc. Depending on what you code, it might save you 30 minutes a week, or might save you 6 hours a week.
Most employers will supply you with an encoder (Codify for me). I use it often to check NCCI edits, whether an assistant surgeon is allowed, and for LCDs/NCDs. When I occasionally come across new procedures, it is helpful to read a paragraph explaining the surgery as opposed to one sentence. If my employer did not supply an encoder (never worked somewhere that did not), I might pay out of pocket for one, but find the cheapest one and only the basic subscription.
Most have free, short, trial subscription so you can see what they are like.