As I understand it, PTRACE_TRACEME causes the process to be put into "traced" mode, where signals (and exec* calls, which will cause a SIGTRAP to be sent) cause the process to stop. This is useful in the context of a debugger because the debugger process will fork, the child will call ptrace(PTRACE_TRACEME, …) to put itself into "traced" mode, then exec the process you want to debug and be conveniently placed into ptrace stop at the first instruction to be executed in the new binary.

The problem with PTRACE_TRACEME is that if you do it to yourself and recieve a signal, you're put into ptrace stop there's and there's no way out of it unless the parent knows how to get you out of it (using ptrace, of course). Sending signals will not work, even SIGKILL; somewhat humorously, on iOS, if you attempt to do the equivalent (using the similar PT_TRACE_ME) and oops yourself, the entire system will slowly grind to a halt as it tries to (at least, I think…) SIGKILL your process for a variety of reasons and fails, at some point locking up waiting for process termination.