It's often entered by hand, by a salesperson with a telephone handset in the other hand. Typos, inaccuracy abound, which limits the ability to analyse and gain insight from the data, as well as to use it effectively in automated processes.
Hubspot have been releasing features and changes that start to address this, and I expect powering it all with Clearbit data will be the first thing they do with this acquisition.
There are quite a few companies competing in this space, so there are options if Clearbit shut the doors to use outside of Hubspot.
My company is one of those, focusing on IP->Business matching, but also selling direct access to our database of business information in certain circumstances. (www.leadforensics.com).