An email marketing provider typically hosts several companies in one email server. Your IP address is usually a part of this pool. A dedicated IP allows you to use a private IP that will be only associated with your sender address.
A private or dedicated IP gives you control over your sender reputation. Though the quality of deliverability solely depends on your practices, a dedicated Ip ensures a significant increase in reputation as compared to shared IP pools.
High volume senders who require IP whitelisting before sending regular emails, benefit significantly from having dedicated IP addresses.