Why do maintenance plans exist as a feature?
Underlying SSIS task that do not require SSIS to be installed to run, nor require you to edit via BIDS or SSDT-BI.

They are there more or less for easy deployment of basic database maintenance that does not require much knowledge to build. Is it something that fits any environment? I would say no, it is mostly for the one man/woman shops that just need something up and running quickly and is sufficient.

Now that is not to say in large environments maintenance plans don't have a purpose, just find they may not used as often. You might consider them a stepping stone to SSIS packages.