Load balancing is needed if you have multiple servers or a cluster running same service. Load balancing will help your organization to access mission critical services without delays. Load balancing can be setup using many operating systems and services including Microsoft.
High Availability is very crucial for mission critical services. High available systems will ensure information systems availability of your organization. For example, you can setup a high available messaging system by configuring an Exchange server cluster. Having this will avoid single point of failures.