The main reasons you want to use ISBoxer imo:
1. Healing with shaman will be really easy. You can heal with shaman while controlling from warrior and you will be able to heal other players. To heal other players with other software while not controlling from healer is something that is very hard to setup (but not impossible).

2. Built in FTL (control from any character and other characters assist/follow that character). Not a huge deal when only playing 2 characters but still useful.
The main reason you would want to change who you control from is that for dungeons you probably want to control warrior, hard to tank when you don't directly control the character.
When you pvp you might want to control from shaman and just spam IWT on warrior to stay on targets. This will allow you to kite with shaman.
You can setup this in most software for multiboxing but takes more effort.

3. It is really easy to use ISBoxer. Other software is harder to use since you need to do a lot more to replicate the built in things in ISBoxer.
I kinda like programming, so I choose to not use ISBoxer and come up with solutions to what I want to do myself. I like doing that but for other people I would just recommend them to use ISBoxer.