Can I use my computer while defragmenting?
W10 currently defragments mechanical disks when it detects that they are not in constant use. SSDs do not require defragmenting.
It is always advisable to let it finish defragmenting.
Because, for example, if you are working with a video application, the disk access is very intensive and therefore slows down the process of reading and writing the defragmentation.
Which both tasks would make use of the disk and in the end the computer would be slower in its performance.
It's like if you're doing a renovation in your house and while the workmen are working, you have one person constantly cleaning. It's a job for nothing.