You need the domain to allow visitors an easy way to access it. However, besides this purely functional part, the domain also fulfills other purposes.
Most importantly, it is part of the branding of your website and company. Makes you more unique and memorable. For example, when you type pepsi.com into your browser bar, you are fully expecting to access the website of the organization that manufactures the soft drink Pepsi.
In addition to that, having your own domain name also gives you credibility. Where the customer will feel that the commercial activity that he deals with has more seriousness, an activity that has its own domain and has its own e-mails from which his e-mails come, not like any normal commercial activity.