For me it's definitely a must. There are plenty of reasons why this is the case and I have been seriously training for about 8 years now. I still have holidays from training and I enjoy food and wine as much as the next person but following the 80:20 rule for the majority of the time I have gained the biggest results by having a training partner.
The benefits I have experienced are:
1) Accountability - some days I wouldn't have trained if I hadn't had the reminder text from my training buddy. There never any pressure to go but if the text comes in then it's on. If I get the text out first he's in.
2) Quality of workout - I have worked out on my own and now and again I'll cut the odd corner or two, it's human nature but when I am in the same room as my training buddy I never cut corners because I know there would be a tut tut from him.
3) Quantity of workout - It's important to note, quality should always come before quantity but saying that half a workout is only half a workout. Adding in the extra core work necessary to stabilise during lifting movements is crucial. How may times have you skipped the core work because you were tired and ready to go home? Having that person there to nudge you along really does mean you will do the work you agreed to when you designed the programme.
4) Motivation - Often, as mentioned we do the minimum required or even less and cut corners. With a training partner that is less often the case. Having that voice of encouragement in the room with you to push that extra rep or to add more weight (if appropriate) is less likely to happen training alone. Gains are made by challenging yourself. As per the picture if someone has the bar you're lifting then you don't need to worry about failing, you can do your best on the day. Progress comes from being batter that you were last week.
5) Personal Trainers - If you have very different goals to your friends or can't find a friend that wants to train with you, you will achieve the same advantages of a training buddy in a personal trainer. They are there for you in the same way. They probably go further than your training buddy would and keep in touch when not training to ensure you are doing your homework or eating the correct things to get you to your goals sooner.
6) Group Training - And that's why group training is so successful. Large or small you are sharing the experience as a team. You can add the odd words of encouragement to each other, share the moans as the hard work takes its toll! You don't want to look stupid in front of a group of people so you just get on with it. Would you do this on your own?
If you want results these are a good place to start.