2bearstudio on "What database structure is this?"

Recently take over a maintenance work on a new WordPress website. The database is unusually large. Observing in phpMyAdmin, found that, the database has 499 tables. Almost every table has multiple duplications, just with different suffix.

For example:
For wp_users table, it has one table called:

wp_users

then followed by another 15 wp_users table list like this:
[+] wp_users (15)

Click on "+", the 15 table names look like this:

wp_users_20121214_020503
wp_users_20121214_020503__20130917_0__20141109_014715__1
......
wp_users_20141109_014715

Any idea what is this, and how could this happen, and how did it happen? Can the table with (date) suffix be safely removed without taking down the site?