So why do these offers exist?

The primary goal of the offer is to get your vehicle into the workshop and your details on their marketing database, from that moment on, whoever is making the offer, need to upsell items and labour to you, to make the transaction financially viable for them.

The usual result 

a young man listens on the telephone with an annoyed expression on his face

You end up paying for a $99.00 service + extra money for additional items that most reliable Automotive Businesses already include in their service quoting any way.