"Why would I use your software instead of a service like Constant Contact?"
There are definite conditions when a email service provider (ESP) would work better for you than having your own email marketing software in-house. Here are some situations you might want to consider when determining the best path for your company:
List size - If your list size is relatively small, in the thousands or ten-of-thousands, and you don't mind giving out your list to an outside company, an ESP is a good solution. You will be able to upload these email addresses and/or set up code on your page to ask for new email addresses. Once your list starts exceeding 100,000 then you may want consider bringing things in-house.
List availability - Does your list change? Is the list actively worked on in real-time? If so, you may find it hard to manage these situations using an outside solution. You will have to create some way of getting new list entries to this provider to be included email projects. It may take creating output files from your CRM or other source and uploading them at a regular basis.
Cost - An ESP is simple and easy to get started with. They usually charge per month based on the number emails you typically send in a month. It is sort of like a Cell Phone bill though where you buy a certain plan and if you don't use the minutes, you lose them. If you go over, you pay for the extras usually at a higher rate.
Privacy - If your email list does not have privacy issues than an ESP will work. If you believe your ESP has good protection and a good reputation you can sleep better, but it could be possible for your account to be compromised or, worst, they ESP could be compromised. Could this happen to you if you have an email system of your own? Sure, but at least you have the ability to add all the security you require.
Having your own emailing system in-house will be more effort to maintain than using an ESP. ESPs can get quite expensive every month if you send large emails. So there is a lot to consider when making your choice.