Does importing specific class from file instead of full file matters?
It depends, in the tutorial that was probably done for readability
Usually if you use most of the classes in a file, you import the file. If the files contains many classes but you only need a few, just import those. 
It's both a matter of readability and optimization.
Most of the tutorials and books about Django or Flask import specific classes from files instead of importing the whole file.
For example, importing DataRequiered validator from wrtforms.validators is done via from wtforms import validators instead of importing it via import wtforms.validators as valids and then accessing DataRequiered with valids.DataRequiered.

My question is: Is there an reason for this ?

I thought to something like avoiding the loading a whole module for computation/memory optimization (is it really relevant?) ? Or is it simply to make the code more readable ?