Putting a photo on your profile does a few things:
- It distinguishes you from other profiles that only have a generic image
- People who look at your profile will relate to you better, as the profile now has a face
- When meeting face to face, people may recognize you from your photo
- It shows that you are who you claim you are
- People who do know you are more likely to connect with you - they know it is indeed you!
The major reason against it is possible loss of privacy.
You need to weigh the pros and cons for yourself.