It's absolutely necessary in terms of you analysing your own play and identifying, correcting mistakes, analysing win rates, and etc. Without this I think it's gonna be an amateur game which is highly reliant on luck and variance. You can download any hand histories to HUD and do your work, there are free HUDs which allow people to do this.
As for the necessity of using HUDs when you're playing, well, you need to pay attention and write notes. HUD is not a panacea, one still needs to work off the tables and analyse stats and learn how to interpret them in total and how to adjust then. Otherwise, a quick look on stats can give you some quick idea of who you're playing against, but not more than when you actually pay attention to your table and your opponents.