sitecore_analytics_index stores information about interactions, contacts, and some other xDB entities. The more data you collect, the bigger the index will grow. The more visits you have in a given time period, the more frequent index reads and writes will be.
Whether or not you should switch from Lucene to Solr depends on whether the high hard disk activity is a problem for you. If you believe that the performance of your CM is slowed down because of this Lucene index, then you should definitely try to migrate the index to a Solr instance hosted on a separate server. This will certainly take the burden off the CM.
Switching to Solr is not necessarily the only solution. Sure, the official documentation says you should upgrade if you have more than 50k documents in the index. But you can also try to switch from a spinning disc HDD to an SSD—this will make disk reads and writes much faster.