What is the use of different blender speeds?
There are many reasons for having a blender with selectable speeds:


Not every time you want to liquify or make a mush of your food; quite the opposite, usually you want to keep a bit of texture into what you are blending.
Thus, lower speed is better.
Blending heats up your ingredients, either by the friction of the blades and by the heating of the blender engine itself. Now, while it's pretty difficult to ruin oils or many other things with the blender temperature, vegetables and fruits on the other hand are really sensitive.   

So, as a generic rule of thumb, you always want to start with the lowest speed and then raise it up step by step (my blender used to have 5 speed settings, for examples) just as necessary. No point in running the blender at max speed by default.
The blender's engine is built to blend things, and as obvious as it may sounds is maybe less obvious that the engine is not expected to runs free blending the air, because it can quickly burn and ruin itself: it actually needs (and expect) some kind of resistance from the food it's blending.
So, when you blend big and/or hard things, it's advisable to do some on/first (this is the reason for the pulse mode), then raise the speed, to not overheat the blender and to not give the blades too much energy. 

For example, if you are blending chickpeas, as they are pretty heavy and big. When you  immediately go at the highest speed all you will get will be chickpeas being struck by the blades and just bouncing all around. When you lower the speed, the impact between the blades and the chickpeas will be less energetic and the blades will have a chance to at least begin to scratch them ;-)