Weight training can offer several specific benefits for all sports but let's have a look at mountain biking.
All of the following attributes will best be put together in a well-planned program so it is best to consult with a personal trainer to get the best out of any program. This isn't a complete list but will cover most of the main points.
Power and Explosiveness:
These improvements will not only make you faster, safer, and more resistant to injury. But it will keep you riding more often and for longer. Both in the short term and the long term as lifting weights is a massive tool to be used to keep you active as you get older.
Remember that a well-rounded training program should also include specific skill and technique work on the bike, cardiovascular conditioning, and flexibility training. It's advisable to consult with a personal trainer to design a training program that aligns with your individual needs and goals.
Also if you want to go for a ride in Scotland give me a shout, I end up in the Tweed Valley most weeks.