Up until quite recently the fully flexible ticket bought you a lot that you could not get with other tickets.

In First ex UK
* Until a few months or so ago the choice was either fully flexible (F) or no changes, no refunds (A). So even if you had a chance of changing the flexibility was valuable.
* Now of course A class fares are changable and refundable with a GBP 200 penalty in each case. It looks like BA took the opportunity to give the fares a hike too...

In Club World ex UK
* The choice until a few years ago was either fully flexible (J), changes with GBP 200 penalty and 20% refund penalty (C) or no changes, no refunds (D and I). If you needed to change it had to be J or C and if you needed to refund the 20% penalty on C was a good incentive to upgrade
* Then it became fully flexible (J), changes and refunds with GBP 200 penalty (C and D) or no changes, no refunds (R and I). The severity of the refund penalty with C and D disappeared and fully flexible tickets lost some of their value.
* Now it is fully flexible (J), changes and refunds with GBP 200 penalty (C, D and R) or no changes, no refunds (I).
* BA has taken advantage of the opportunity to hike the upsell between I and R a considerable amount as a result of this change...

So in the past when BA.com was first getting seriously going as a major e-commerce operation there was a clear distinction ex UK between fully flexible and other. It made sense to make the distinction between fully flexible and lower priced. Now however this is no longer the case yet the fully flexible box remains, presumably for familiarity reasons and also to fool some people into buying J or F even if they only need R or A.

For further details see my thread:
CW and F fare structure/rules changes from UK