Many security problems arise from the simple fact that you must allow user access to your systems. In any event it is all to possible for users to attack a system, so it is advisable to monitor users, and limit the amount of damage they can do. Limiting users Users require resources like CPU time, memory and drive space to do their work. On many systems it is possible for users to hog resources, reducing the usefulness of the system to other users or in some cases actually bringing the server to a slow halt or crashing it.
Programs can crash.
If you do not allow root to run enough processes for example cron jobs may fail or you may not even be able to log in to fix any problems. Almost all Linux distributions ship with PAM support making it universally available. As well applying limits selectively through the command shell is very difficult, whereas with PAM applying limits globally, on groups or on individual users is quite simple. Maximum number of open files. The priority to run user process with.