Having a log in area for your customers can be very useful. Many websites have log in areas for different reason. It may be a simple way to store information such as name, addresses and possible order history or you may need a more advanced system where they can change subscriptions or memberships, upload and download documents and send secure messages.
An accountancy firm for example, may well need to have a secure area where they can share documents with their clients. These files will be strictly confidential so the login area will need to be encrypted correctly.
Some businesses may give access to a staff portal to employees who want to be able to view their payslips or possibly to request holidays. This can cut down on admin time and also on the amount of paperwork that needs to be printed. Often portals take away the need for printing and postage so are extremely environmentally friendly
You may have a client portal by a third party which is accessed through a different URL. If this is the case then it is a good idea to get it to open in a new window so the visitor can return to your site after they have logged in to the third party area.