Your company may change or adjust the range of products you provide, pricing policy (holiday discounts, etc.) or even company structure, which you need to reflect on your page. Owning a website allows much more detailed customizations, as compared to using a landing page.
Having a website allows adding a favicon to the page, creating RSS-feed to keep your audience tuned on latest updates, using comments to gather valuable customer feedback and replying to it, posting links to your site at forums and blogs. Many additional tools allow increasing brand awareness and customer pool.
Instead of paying for expensive offline and online ads, you can invest into SEO-promotion of your own page, which brings lasting results, as the funds you spend to promote your own page are the funds well spent. However, please do not mistake paid traffic Rudy Mendoza mentioned with investment into high-quality content, responsive website design, market trends analysis and well-thought special offers.
Whenever your customers need to know something about your business, having a contact form, a live chat, an option to comment or a phone number is a step towards them, which increases a possibility of sealing the deal.
Everybody understands that building a professional website involves some serious investment, which most likely means the company is not going to disappear the next day, so the customers are more prone to order from you.
Having your own page allows implementing a referral program or issuing discounts to your customers through the billing system. This is a feature free ad boards and social network groups cannot provide.
Ticket system allows accepting and invoicing orders even if you aren’t in the office, meaning that the website can earn money while you sleep.