Custom Offers is a feature, which allows sellers to be proactive and offer their services to potential buyers in a customized manner.
By sending buyers a Custom Offer, you have the flexibility in pricing. You can conduct larger deals and offer higher prices - there's no need to start at $5, you can set the price. It also allows you to accommodate specific buyers' requests with a personalized offer.
When buyers contact you and inquire about your service, you can just redirect them to your project page. However, you're likely to lose them along the way.