These boards are used if the players want to know at the start of the game all of the Epoch III Techs that may come into play. The turn 7 board contains the 5 Epoch III Techs that will be available on turn 7. The number of Techs on the turn 8 board that will come into play is dependent on how many Techs are purchased on turn 7.
For example, if 3 Techs are purchased on turn 7, 3 of the 5 Techs on the turn 8 board will be put into play. We shuffle the 5 turn-8 tiles and select them at random.
Again, no official rules provided so we can only guess whether
- boards are populated at the start of the game or just at the start of Epoch 3.
- Turn 8 Techs are only available if the Turn 7 techs are purchased (i.e., it forms a queue) or all unpurchased Turn 7 Techs are discarded at the end of Turn 7.
I agree that revealing them at the start of the game is more strategic although obvious downside is that it is yet another thing for new players to try to remember. (Maybe a compromise would be to reveal at the start of Epoch 2)
Just a word of advice, if you do make Turn 8 Techs only available when Turn 7 techs are purchased (i.e., as a queue), you need to explain this 2 or 3 times as otherwise some players will either not fully understand or forget and assume that all Turn 8 Techs will be available and then shoot themselves in the foot.