It's quieter.
The pistes aren't deserted exactly but there are minimal lift queues, optimum ski conditions (due to the typically colder weather), and you don't have to contend with the school holiday crowds. The whole mountain feels easier to navigate. For many skiers that will mean the joy of more skiing and less waiting.
Sometimes the lift pass companies do special deals on January skiing, so it's worth taking a look at what is on offer.
Also the ski schools will have more availability whether it's a group beginners lesson or private instructor you're looking for. And the equipment hire shops will have more stock to choose from, and their staff will have more time to discuss with you any rental requirements, help you with boots, skis, boards and any changes you need during your stay. We'd still recommend you book as early as you can for these… it's quieter yes, but ski schools and hire options can still fill up.