Most companies that are based on intellectual property, like software companies, need to maintain ownership of the copyright so that they can continue to sell the IP (software) for money. If someone else owns the copyright, the company has no way to prevent all the source code being released in the public domain, available to everyone for free.

If you don't own copyright on your own software, you'll also find it hard to sell the business in the future. Potential acquirers are usually interested in purchasing the IP owned by a company, not just the company itself.

IP that a company owns can be counted as an asset when drawing up a balance sheet. It is often quite a substantial part of the assets of any major software company, making the net value of the company much higher.

In my experience, companies usually default to owning the intellectual property for everything an employee produces then provide a way to waive their interest on outside projects by employees. The IP waivers require the employee to specify the scope of their outside work and get approval from the company directors so that the company's IP rights are waived on that code.