The R-70 has better interference-fighting controls than the '720 does. With the EX-7072, you can control the '720's operating frequency by means of the R-70's tuning dial. It's like having two receivers in one.
Go here and read N3WXW's review of the setup:
http://www.eham.net/reviews/detail/2594?page=1
An eBay listing for one: https://www.ebay.com/itm/Icom-IC-707...-/192093772904