Coverage allows you to beat more threats. Gliscor is a prime example of this; without Ice fang, it wouldn't stand a chance against Dragonite, Terrakion and Exca [when it was around].
But a lack of coverage hinders its ability as a wall, besides, if you can constantly force out Mag or at least accumulate damage on it, it helps a ton, given that Magmortar is one of the best counters to Tangela and is threatening in general.
Hi DTC, to respond to your Tangela post, Leech Seed is adorable and all, but Tangela loves to accumulate that extra damage to Swellow/Mag with HP Rock when they switch in and take SR Damage. In addition, Tangela can just switch in and out to get back its health and just whittle down the opponent's Pokemon's health with Giga/HP. HP Rock and Ice are extremely underrated in what they can beat, whereas Tangela doesn't have to independently rely on leech seed to take down most mons, where it does to take out something like Altaria with HP Ice.