Several applications need random numbers for correct and secure operation. When ssh-server gets installed on a system, public and private key paris are generated. Random numbers are needed for this operation. Same with creating a GPG key pair. Initial TCP sequence numbers are randomized. Process PIDs are randomized. Without such randomization, we’d get a predictable set of TCP sequence numbers or PIDs, making it easy for attackers to break into servers or desktops.
On a system without any special hardware, Linux seeds its entropy pool from sources like keyboard and mouse input, disk IO, network IO, and any other sources whose kernel modules indicate they are capable of adding to the kernel’s entropy pool (i.e .the interrupts they receive are from sufficiently non-deterministic sources). For servers, keyboard and mouse inputs are rare (most don’t even have a keyboard / mouse connected). This makes getting true random numbers difficult: applications requesting random numbers from /dev/random have to wait for indefinite periods to get the randomness they desire (like creating ssh keys, typically during firstboot.).