There are several reasons for using a landing light. Or, essentially, any aircraft light.
- To see what's in front of you at night (duh)
- To increase your visibility for other aircraft
- Signaling of take-off/landing clearances to ATC and other aircraft close to the runway
- Reduction of bird strike risk
And there are reasons for switching lights off:
- To protect your night vision/not get disoriented yourself (strobes in cloud!)
- To do the same, but to other airfield users
- To save money: a hot landing light filament is supposedly more vulnerable to vibration than a cold one.
Except for the first one all are subject to debate and people do have different opinions on them - even instructors. And since the use of a landing light for GA VFR flights is not subject to any law or regulation, expect a healthy debate every now and then.
Me - I turn them on whenever I feel there's even a marginally good reason for using them. That includes all circuit/runway operations, and whenever I get a traffic warning.