Registering a domain in multiple extensions can be beneficial in certain situations, but it is not mandatory and is not always necessary.
One of the advantages of registering a domain name in multiple extensions is that it can help protect your brand name and your online presence. For example, if you have a website named "example.net," it may be beneficial to also register the domain name "example.com" to ensure that no one else registers this domain name and attempts to hijack your online identity. Additionally, using multiple domain extensions can help increase traffic to your website.
On the other hand, registering multiple extensions can be costly, and you may need to renew each extension separately. It can also be difficult to manage multiple domain extensions and keep them up to date.
In conclusion, registering a domain name with multiple extensions can be beneficial in certain situations, but it is not necessary or mandatory. If you choose to register your domain name with multiple extensions, make sure your budget allows for it and that it benefits your online needs and goals.