Fifteen years ago there weren't plenty of other packages that offered process monitoring: There was init (inittab), /etc/ttys, and roll your own (init.d aka rc.

Fifteen years ago there weren't plenty of other packages that offered process monitoring: There was init (inittab), /etc/ttys, and roll your own (init.d aka rc.d). People didn't use inittab or /etc/ttys because telling programmers to edit these files was error prone and if they got it wrong, the system wouldn't boot up correctly. init.d didn't have that problem, but programmers so frequently and regularly and consistently got "init.d" wrong in some other way (supervision, logging, etc) that some time in the last fifteen years most unix sysadmins stopped expecting programmers to daemonize and write pid files and simply started monitoring their services for them with tools like daemontools.