Any employer that offers an occupational scheme must usually also offer staff the chance to save more if they wish through AVCs. (Sometimes an employer may make a contribution as well, though this is very rare.)
Your AVCs are normally used to buy an additional 'money purchase' pension. It is like having an extra personal pension on top of your occupational scheme. Because an AVC pension is arranged through your employer this may well have lower charges than an ordinary personal pension. You can have both a stakeholder pension and AVCs. AVCs are usually a good way of topping up your pension.