If the electoral college ties 269-269 (unlikely but possible), the Presidency is decided by the newly elected House of Reps delegations in each state voting in something called a "Contingent Election". 26 states needed to win, majority rules within each state.
These 8 close House races are the Blue Wall for the makeup of their close state delegations (toss up House races in other states like CA/TX won’t matter for this). If Dems lose any of these 8 elections, the Republicans will likely have a majority of the Representatives in 26 states.
Other reasons these 8 races matter:
—Dems need to net five House seats to take it back.
—7/8 have follow-on impacts for the Presidency or important Senate races. A Dem House candidate having more $ for advertising or door-knocking helps the other Dems on the ticket.