A full suspension mountain bike (often called a 'full squish') is the right choice when you're riding rough, technical terrain with lots of rocks, roots, and steep descents. The rear shock keeps your wheels tracking the ground, giving you significantly better traction and control compared to a hardtail. If you're hitting enduro trails, bike parks, or long descents with chunky features, full suspension will keep you more comfortable and confident.
The trade-off is that full suspension bikes are heavier, cost more, and require more maintenance with additional pivots and a rear shock to service. However, modern designs have come a long way—pedaling efficiency on climbs is much better than it used to be, so you're not sacrificing as much climbing performance as you might think.
If your local trails are smooth and flowy, a hardtail could serve you just fine. But if you're spending hours in rough terrain or want maximum comfort on long rides, full suspension is worth the investment.