While using an ESP to send out your e-communications, there are two options in regards to the IP setup for your account.
1. Shared IP
A shared IP address will be particularly useful in regards to maintaining sending reputation if you are sending very low volumes or low frequency. (E.g. you are sending only monthly communication).
In fact, while your account is being setup on shared IP(s), assuming that you are setup with senders of the same kind of sending reputation, this will help you ensure a correct inbox rate of your communications even though you are only sending communications once a month thanks to other senders which are maintaining a good IP reputation.
A shared IP address could also become a problem when one or several other advertisers being setup on the same IP(s) as you start targeting the dodgy email list, generating low engagement and potentially high complaints therefore dragging down the IP(s) reputation which will surely impact your communications. Such issue might become dramatic for your business if most of your sales are being made through the email channel as your emails might start to be delivered to the junk box, therefore lowering down open and click rates, this last one being the source of your revenue.
2. Dedicated IP
A dedicated IP will be highly recommended if you are sending a consistent volume of emails, at least once a week.
Being setup on a dedicated IP will allow you to build your sending reputation for both your IP and sender. While being setup on your own dedicated IP, you are keeping full control of your sending reputation which will ease the process of troubleshooting any delivery or deliverability issues to ISPs you might be experiencing.
A dedicated IP might also become a problem in the case of an agency sending one-shot campaigns for its clients as you have no way of knowing the origin of the database being provided by your clients, this might be very problematic as some old email lists are full of spam traps or contain invalid addresses which can easily ruin your sending reputation you spent time to build and this can happen with one single dodgy campaign.