Intuit sells tax preparation, bookkeeping, and accounting software to individuals, small businesses, and other organizations. I used Quicken to manage my checking account back in the day, and later used Quickbooks for bookkeeping when I was the head cook and bottle washer of my computer store. I'm now retired into a simpler life and find a spreadsheet to be adequate. Many churches, clubs, and charities use Quickbooks.
MailChimp is a mailing list handler used by a lot of churches, charities, clubs, etc, for newsletters and the like. Small businesses use it for advertising.
Intuit and MailChimp serve the same client base, and have similar needs with regards to customer support, advertising, legal services, etc. A merger makes a lot of sense.