I wanted to know why (and if) is useful to map the reads back to the assembled sequence.
Does this gives validation about the coherence of the assembly?
Yes. More specifically you can evaluate the coverage of the assembly to see that you don't have weird things going on.
Mapping reads back to the genome is useful to check completeness: you would expect ~99% of reads mapped, depending on your sample. Also, it can be used to detect collapsed regions, usually repetitive DNA, by identifying high coverage regions: for example >3 times the average.