Being a diver with less than 30 dives, why should I do an accompanied familiarization dive at the housereef?

For your own fun and for your own safety! Finally, the conditions are different everywhere, maybe you have a new piece of equipment or didn't dive for a longer period...
With such a familiarization dive you've got the opportunity to test the proper amount of lead and you get used to the equipment in a relaxed way. In addition, you have a professional with you, who can show you many interesting creatures.