What advantages does in-memory OLAP have over traditional systems with significant memory? Do in-memory OLAP engines have advantages over the traditional OLAP engines backed by enough RAM to contain the entire cube(s)?

For example, if I use a MOLAP engine (SSAS) and GB / TB of RAM where the entire cube (or even star-schema) is RAM resident, what is the difference compared to something like TM1 / SAP HANA?
Not my field of expertise but as I understand it the difference in the majority of so-called in-memory OLAP databases (not a term I'm fond of, it's used as marketing pitch more than as a fair comparison of technologies) is column store indexes.

Column-Stores vs Row-Stores (How Different Are They Really) is a good intro to the technology if you're familiar with traditional OLTP and OLAP database structures.