A slide joint couples two members while allowing them linear motion along a single axis relative to one another. In Transformers toys, this motion must sometimes be partly restricted to allow joints to remain in one or more configurations. This restriction is often provided by friction or by a sprung tab that fits into a notch.
When a slide joint allows one member to slide into and be partly or wholly concealed by the other, the joint is said to telescope, after the similar joints of a collapsible spyglass.
Slide joints can appear as a convenient way to hide some robot body parts while in an alt mode (as opposed to, say, removing the fists to store elsewhere). This would also help with limb length: many toys 'extend' a part to let legsform from some other part.
Blitzwing has two sets of these in pairs. One is his jet thruster/tank front (?)/leg, which only actually extends in robot mode. The other set is his fists, which again only retract in robot mode. For his legs, it allows him to get the appearance of long normal-proportioned separated legs while hiding the excess length 'internally'; for the fists, this also helps in storing his arms when the fists aren't needed, as they retract into the arms and don't end up appearing when he's in tank or jet mode.