At 3 cost to digivolve from a LV3 into Lobomon and then to the LV6 AncientGarurumon, it costs the same amount of memory to field 1 rookie into battle. But instead of being only able to target security (due to rookies having low DP), AncientGarurumon can target an opponent's Digimon or overcome an opponent's blocker. On top of that, AncientGarurumon can by-default perform a minimum of 2 checks (by unsuspending itself). In addition, the player can summon for free a LV4 hybrid digimon to replace the deleted AncientGarurumon at the end of the turn.
To take advantage of this combo, hybrid digimons are used extensively in this deck. We all know that hybrid digimon can evolve from Tamer, we can evolve them to BT4 Lobomon and attack security before evolving LV3 digimon to promo Lobomon (as long as the tamer is not play in the same turn)