First, important evidence and witnesses can disappear. A lawyer can help preserve testimony has access to experts who can preserve evidence for use at trial.
Secondly, an attorney has training in knowing what damages and statutes of limitation are applicable to your case. If you attempt to resolve your legal dispute without an attorney you risk leaving out certain elements of damage and possibly missing the deadline for filing suit.
Third, an insurance company or opposing party may take your claim more seriously if you have an attorney. If they know you have experienced legal assistance, you may be able to resolve the dispute or claim without going to court.