Having a website created or updating can be exciting. You will often sit down with your programmer / designer and talk about what you want. They will usually then go away and either create visuals for how the website will look and function or they will give you a detailed description of exactly what will be included on the site.
You may feel that once you have agreed on the specifics you no longer need to keep in contact with them unless you need to ask them something, but losing contact with your designer / programmer can lead to a number of issues later on down the line. It may be that you have your website built, it goes live and is just left to tick over for a few years. After this time you may want to make some changes to the site, but find that you can no longer get in touch with them. This can cause a number of issues as if you do not have access to the hosting / domain names you may struggle to get control
Often a web developer will need FTP access to your site in order for them to make changes to it and the only person that will be able to give them this is the developer who is hosting the site.