They have a feature called ALIAS records, which is a non-standard DNS record type similar to CNAME, but it works with the root domain. The people I know using DNSimple were using it because it worked better with Heroku than other DNS providers at the time. They are not the only ones offering ALIAS records now, but it's still a non-standard feature that not every DNS provider has. A feature like this is needed for Heroku, since they do not give you IP addresses for your servers - you have to use CNAME or ALIAS records.