Unlike your offline store (and you, human), your website is always awake to receive visitors. Think about a salesperson who never rests 24/7, all year long.
People have a strong bias toward businesses with websites. A website tells them your business is real and won’t run away with their money.
A website puts you firmly above your competitors if they don’t have one. You can amplify your winning unique selling points if your competitors have websites.
Your physical office or store is limited to a number of people. A website can open you up to the world and expand your business to a global audience.
Your voice on social media is restricted by the platforms’ character limits, policies, etc. On your website, you can say anything about your brand, it’s your turf.
Including a portfolio, gallery, or case studies on your website helps people decide to work with you. You can also add testimonials from past clients.