A variety of benefits that the plant might get from its thorns have been proposed here.  Although these all have some plausibility it seems most likely to me that thorns initially arose as a defence against herbivory by mammals (as has been pointed out they are not much help against insect herbivores which are better deterred by either dense hairs, thick leathery cuticles or by biochemical defences).  Some of the bramble consumers that Oliver Lehman mentions eat the fruits  (which the plant benefits from as this helps spread the seeds) but are not really leaf eaters.

Of course a feature can evolve and then provide secondary benefits which may be quite different from those that lead to the original selection of the trait.  In the case of brambles and other climbers/scramblers it seems very likely that the role of the thorns as a climbing aid provides a selective advantage in this way.  My guess would be that the 'carnivory' theory is unlikely to have played a significant role in the evolution of the thorns but may well provide an occasional side benefit.

I believe it was made quite clear in the time lapse footage for the BBC TV show "Secret Life of Plants" that the swaying motion as the stalks grow were seeking out places for the thorns to latch, thus giving the extreme lengths much needed purchase and stability ready to bear the weight of the fruit to come...

The evolutionary advantage may be that a thorn that catches and (with higher probability) kills the predator means that this predator will not go on to damage the plant another day.  This increases the plant's probability of reproduction, while lowering that probability for the predator.

I'm guessing thorns attract insects to improve pollination. But thorns also repel predators to extend their life.

My comment about carnivory was made light-heartedly - but if anyone publishes a paper in Nature, you all know where you read it first.