Tarantulas are capable of spinning different types and amounts of webbing for different tasks. However, generally speaking, they only lay 'sticky' webbing when they actually need to anchor it to the ground, rather than true spiders, who use sticky webbing extensively to catch prey. Likewise, tarantulas do not try and hang off a single strand of webbing like some true spiders, most likely because they are simply too big and heavy for it to support them.

However, without doubt they use webs well, and they still serve many and varied functions, some of which are listed below:

Feeding Mat

Spiders will usually spin one of these every single time they feed, unless the prey is simply too small to warrant one. They serve multiple purposes including providing a sterile and clean surface from which to eat, separating food from substrate, and they allow easy attachment of 'security webbing' while they wrap up prey.

The size of mat depends entirely on the size of the prey, and the amount of them that require storing ! Which is their other main function - if a spider makes several kills in a short space of time, it will wrap others and then consume them one at a time.

Sometimes feeding mats are cleared away after a meal, but sometimes not. Occasionally, a spider may add to a previous one.

Moulting Mat

Also spun without fail every time a spider moults, these webs are much thicker and more dense than feeding mats, and in some arboreal species are made almost like hammocks or cradles, in which the spider may safely turn over onto its back, ready to moult. They are usually dismantled and wrapped up shortly after the spider has finished with them.

Trip Lines

Spun across sometimes vast distances, spiders use these to detect movement from afar, or at least from outside the immediate range of their senses.  It can probably be assumed that all spider webbing transmits vibration information back to any part of a spider that is touching a part of it, but trip lines tend to be single, or few-stranded threads that join to more extensive 'mat' or burrow webbing further down. The spider will often rest its feet and pedipalps on these trip lines.

Security Webbing

Immediately after laying down a feeding mat, most tarantulas will lay their recently acquired meal down on it and perform a small amount of 'restraining' webbing, the function of which is not entirely clear. Often it is thought that they do this to restrict the kicking legs of whatever prey they have caught, but in actual fact the spider usually waits for the food to have stopped moving completely (when it is presumed dead) before it applies this. However whilst it could be a safeguard used this way, I prefer the theory that this webbing keeps the food in a loose bundle whilst it is being consumed, and makes it easier for the animal to manipulate. It may also minimize clearing up time afterwards. Most tarantulas 'clear up' after themselves, and some will further wrap the remains, and deposit them in a chosen corner of their enclosures.

The 'other' form of security webbing is more general, and is there to keep the spider advised of what else is occurring in the tank at large.

Burrow Lining

Most tarantulas feel the need to involve webbing somehow in their burrows, although to wildly different extents between species. Chromatopelma, Heteroscodra and Avicularia are well known to web extensive, and thick home webs, whilst others seem content to be touching the substrate in their homes (Lasiodora, Lasiodorides, Aphonopelma). Other species (of mine) are somewhere in the middle (Poecilotheria, Psalmopoeus, Ephebopus, Acanthoscurria) and may line their underground burrows with varying amounts.

Species that can flick urticating hairs will often incorporate these hairs into burrow webs, most likely as a deterrent to other animals to enter.

Burrow Sealing

For those spiders that want to moult in their homes, almost all will use thick and insulating webbing to seal themselves into their burrows shortly before a moult.  They simply tear it down when ready to emerge.

Mapping

Like all spiders, true, or otherwise, tarantulas also use webbing to map out their environment. Initial explorations of tanks always feature this type of webbing, and they often use it on a daily basis as well, assumedly so that they know where they have been.