When registering you may get additional functionality of the product. Some companies use this as a means to enforce licensing of their products.
Many software companies use the registration to create push lists for patches, updates, and important notifications.
Registered users often recieve notifications of special offers and promotions that may be of interest to them.
Sometimes software companies will provide free gifts to those who register as an incentive. The free gift might be addional software or some cheap swag like a thumb drive, mug, or t-shirt.
Additional purposes:
The software company is able to compile a database of current users who are then potential future customers for additional products.
Some companies will share or sell the information with other companies for financial gain (Remember to read the privacy policy statement of the company before you register!)
