When a sender or company used only one IP address for all their emails, such an IP is a dedicated IP address. Such an organization or company does not share its dedicated IP address with another organization or sender. When companies want the full management of their IP sender, they use a dedicated IP address.
Each inbox provider keeps track of the IP addresses of incoming traffic. They also monitor the reputation score for the same IPs. But when you have a dedicated IP, it puts the burden of responsibility for its reputation on you. This is vital because the reputation of the IP address where an email comes from determines a lot about the emails, including whether they are filtered, blocked, or get to the inbox without issues.
Conversely, using a shared IP address, which is pooled among several different senders, can be detrimental considering how you can be affected by the email behavior of other senders. But if you do not have sufficient email volume to support a dedicated IP, a shared IP remains your best option.