Why should one uses IO1 volume over GP3 when it is now possible to manually enter the number of IOPS desired for the GP3 volume ?
It always depends on what your workload needs. The documentation has a nice comparison chart: EBS Volume Types: Solid state drives (SSD)
The main differences are the following:
If you need EBS Multi-Attach or more than 16k IOPS, you'll probably want to go with io1 volumes, otherwise gp3 will usually be fine.