In certain scenarios if you need to modify the schema for a table in MySQL it will lead to the entire table being locked, and for large tables this could lead to a noticeable outage for users if you need to run queries on that table. One case I had where we faced this problem was changing the primary key for a table from 32 bit to 64 bit ints since we were running out of space. We used Percona's online schema change tool for handling this, which wrapped the creation of a new 'ghost' table (which has the target schema you want), rate limited writes from original table to ghost table, triggered writes from original table to ghost table as new writes came in, and finally a table rename from the ghost table back to the original table name in order to perform the full migration with no data loss or outage.

Sounds like this tool is doing something similar but avoiding the use of triggers for flexibility.