Footprint groundsheets are a must have addition to any tent particularly those with integral sewn-in groundsheets. This may sound a bit confusing as if your tent already has a sewn-in groundsheet why would you need another one. The answer is simple. A footprint groundsheet provides a number of benefits;
It will keep the existing groundsheet relatively clean and dry
It is easier to clean than the groundsheet that is connected to the tent
It provides an extra layer on the surface providing insulation and protection from stone damage
Footprint groundsheets are specific to certain tents and thus they are cut to the exact floorplan of that tent. This will aid with pegging of the main tent as the shape will already be laid out for you and you will pitch your tent quicker.
If there is any add-on you should buy with your tent purchase then a footprint groundsheet is definitely the one to buy.