You have to be root or the owner of a process to change its priority.
Reasons to Change a Process's Priority
If a high priority task has a flaw in it, a brute force attack will have more cycles to work with. Because of this, a low priority task is marginally more secure. However, the difference is so slight as to be not worth mentioning.
Administrators can use /etc/security/limits.conf to make a certain user's default priority be higher or lower.