WordPress has a few security precautions set up, but this does not compare with what a trusted security plugin can do. WordPress sites are often targeted by hackers, so it always pays to take additional measures. The best WordPress security plugins provide the following things:

Monitors security actively
Scans files
Scans for malware
Monitors blacklist
Hardens security
Post hack measures
Installs firewalls
Protects against brute force attacks
Alerts if a security breach occurs