Depends what you want to do.
pH Controller for calcium reactor, you don't need a fancy Reefkeeper/Aquacontroller if that's all you want to do. Milwaukee makes a good one. And you can always buy light timers without any issue
ditto on temp or orp controller.
However if you want to do all of them, the price does add up and an all-in-one unit is useful. In addition to the above mentioned items, having it control mechanical switches and what not is nice. Even pump controllability for some pumps on higher end models or with add-ons.
For me, and FYI I'm still waiting for one particular controller to come out before I get one, I'm interested in the history of those various variables, I want to know what the tank is doing when I'm not taking a direct measurement. pH controller? I don't need it as I don't have a calcium reactor, but I still would like to know what it's doing through the day and night.
For a single controller, you're probably looking at around $100. For a "unit" that does the "bare necessities" then the Reefkeeper 2 or Aquacontroller Jr will run you about $300 for everything. (no ORP however).
Are they necessary? Nah, unless you might have a calcium reactor or ozone gener
ator where an "overdose" could pose a hazard to your tank.