To prevent the program from being blocked by a lack of unblocked LWPs.
For example, if there are more runnable unbound threads than LWPs and all the active threads block in the kernel in indefinite waits (such as while reading a tty), the process cannot progress until a waiting thread returns.
To make efficient use of LWPs.
For example, if the library creates one LWP for each thread, many LWPs will usually be idle and the operating environment is overloaded by the resource requirements of the unused LWPs.
Keep in mind that LWPs are time sliced, not threads. This means that when there is only one LWP, there is no time slicing within the process--threads run on the LWP until they block (through interthread synchronization), are preempted, or terminate.
You can assign priorities to threads with pthread_setprio(3T); lower-priority unbound threads are assigned to LWPs only when no higher-priority unbound threads are available. Bound threads, of course, do not compete for LWPs because they have their own. Note that the thread priority that is set with pthread_setprio() regulates threads access to LWPs, not to CPUs.
In particular, a lower-priority unbound thread could be on a higher-priority LWP and running on a CPU, while a higher-priority unbound thread assigned to a lower-priority LWP is not running. In this sense, thread priorities are just a hint about access to CPUs.
Realtime threads are useful for getting a quick response to external stimuli. Consider a thread used for mouse tracking that must respond instantly to mouse clicks. By binding the thread to an LWP, you guarantee that there is an LWP available when it is needed. By assigning the LWP to the Realtime scheduling class, you ensure that the LWP is scheduled quickly in response to mouse clicks.