According to documentation https://www.zabbix.com/documentation/current/en/manual/config/macros/lld_macros

host template in host prototype couldn't be defined through LLD macros.

Without this feature you need to create a separate host prototype for each template. Supporting a large number of host prototypes with different templates is challenging.

Having the ability to set the template through a macro will allow you to create one host prototype, and the value for the macro can be generated, for example, through a script. 
This can be useful when integrating zabix with an external service, which is the source of the truth of the host configuration created in zabix.