Every booking through Airbnb costs you 3–5% in host fees. Booking.com takes 15–20%. VRBO charges 5%. For a $200/night property booked 20 nights a month, that's $600–800/month going to OTAs.
You've built relationships with returning guests, established your brand, maybe even have a website — but without a proper booking engine, you're sending guests back to OTAs to book (and pay commission).
Standalone direct booking solutions exist, but they cost $30–60/month on top of your PMS. And they require separate calendar management, separate guest communication, and separate payment processing.