Why do maintenance plans exist as a feature? Do maintenance plans provide any functionality beyond their component parts? It would seem that the only value added is the convenience of grouping subplans into logical bundles, and the simplified GUI for creation. All the actual work is being done by the underlying SSIS tasks which are being scheduled as agent jobs. Is there anything more to it than that?
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.