One reason for using a load balancer in front of multiple search heads is to enable users to access one of any of the multiple search heads using a single point of access.
Using a load balancer will also ensure access to search artifacts and results if one of the search heads becomes inaccessible.
Keep in mind the following gotcha:
The load balancer must employ layer-7 (application-level) processing.
Configure the load balancer so that user sessions are "sticky" or "persistent".
This ensures that the user remains on a single search head throughout their session. "