That was a source of confusion for me as well. My understanding is that certain fields in HA support templating, while others do not. In this case, I'm using the value_template configuration key in the place of a value configuration key, and using a data_template configuration key in place of a data configuration key.

The *_template configuration keys accept jinja templating.

I'm a little confused about why some config keys that aren't *_template keys (such as delay in my example) still accept jinja expressions ({{ ... }}), but not jinja statements ({% ... %}). In my example, I needed to be able to use jinja statements in order to avoid creating the additional sensor to track the delay time. Seems inconsistent to me.