I have wondered  that myself from time to time and my response is speculative. For one thing, many plants with thorns of that type are scramblers and climbers, and the thorns invariably are so placed that they aid the climbing process and stabilise the grip.However, true as that may be, it is by no means definitive. Many trees, including one-time members of the genus Acacia (currently in the process of being split into multiple genera) have just such thorns. Some of them are what you might call semi-scramblers and their thorns often are rather fine, and incredibly sharp. You go near such a plant at your peril. The same applies to certain species of asparagus in southern Africa and no doubt elsewhere.

As for the protective effect of the thorns, I can only imagine that it is more difficult for selective browsers to avoid such small, affectionate thorns, than to extract leaves from between large straight thorns. A tiny, delicate antelope that could easily retrieve individual leaves from between large acacia thorns, might have a far nastier experience with say Acacia melanacantha var detinens or Asparagus stipulaceus, than with the giant thorns of some strains of Acacia grandicornuta. The victim might well find the risks of feeding from plants with tiny hooked thorns too great to be worth while except in times of drought.

Black rhinoceros might take a different view. This could affect the distribution of various thorny species relative to the occurrence of various classes of browsers.

I've seen lambs so tangled up in brambles that they can hardly move. Perhaps it's actually a carnivorous plant that we haven't yet recognised as such!

It does seem odd, though. The main consumers of brambles are too small to care about the thorns (mites, insects, dormice, birds etc.). The one thing that many thorny plants seem to have in common is forming dense thickets, so perhaps the deterrent effect of hooks helps establish an untrodden area for growth. Bramble also readily roots from the stem, making loops of thorny growth that can be very hard to walk through.

There are some interesting answers but they skirt around the issue of a "carnivorous" plant. It may be that brambles, for instance, Himilaya blackberry bramble patches are indeed carnivorous. It would work for the plants if animals became stuck and die causing the animals to decay and add nutrients to the soil. Also, dead animals would attract other animals that would either become stuck themselves or find a home living in the brambles. That would add even more nutrients. There are many instances in nature of baiting.  Once smaller animals, raccons for instance, make their safe home in bramble patches the bramble plants start enjoying the benefits of the excrement and other nutrient releasing material brought in by those smaller animals.

Birds do a great job of spreading seeds from berry producing brambles. The need is for nutrients as the "seeding" problem is solved by the birds and other berry eating animals that spread both seeds and nutrients via their excrement. 

Roses were among the earliest flowering plants and possibly evolved thorns for defence against broad-snouted megafauna grazers; e.g., with rhinoceros-style noses.  The more gracile giraffe/antelope style snouts came later but the thorns continue to be useful for defence for both the plants and the animals which they shelter.  And because sheltered (and captured!) animals contribute nutrients its win-win.

Just adding this to the discussion, not as an answer - notice that thorns work on fur, not on feathers (or scales). Birds don't get tangled in them. So a bramble could have a very different evolutionary relationship with birds than with mammals.