Is is necessary to use security plugin for wordpress? [closed]

This highly depends on your capabilities as a developer and you are addressing multiple questions in one go:

  • Your servers need to be secured, might benefit from DDos mitigation systems like a CDN or like large hosters offer (AWS, GCE, etc.)
  • Your user(s) input, both frontend and backend, should be validated, sanitized and escaped before going into the DB
  • …

If you have no idea of basic security measurements, then it might be a good idea to enable such a plugin from time to time—do not leave such a plugin active, if it does not monitor every single user activity and DB change, which will be a stupid idea performance wise. Still better would be to get a sysadmin to care about your servers and really good developers to review your applications code.