Maw of Malmortius helps attack damage-based champions against enemy teams with high amounts of magic damage.
Lifeline can absorb most of the initial burst of magic damage from an enemy champion and give the owner an opportunity to attack or retreat. It is very useful against ability power casters with long cooldowns, such as Morgana and Ziggs.
Maw of Malmortius can be good on attack damage-based champions who benefit from being at low current health (e.g. Tryndamere, Olaf). Their low-health passives combined with Lifeline allow them to continue fighting at low-health more safely.
Maw of Malmortius still provides less effective health against magic damage than dedicated, anti-magic damage defensive items like Spirit Visage or Force of Nature. However, Maw of Malmortius come with the advantage of not sacrificing offensive potential, which allows an AD champion to better recover from behind when facing an enemy AP champion. Keep in mind, however, that Maw of Malmortius's passive is not useful at all against most AD champions, while the other aforementioned defensive items still give effective HP and sustain to use against an enemy AD, even though the magic resistance itself is not useful. This can be significant during the mid game if you aren't preparing an armor item against them, or if the enemy AD champion is fed. Choosing wisely between those items can overcome this issue.
Unique Passive - Lifeline: If you would take magic damage that would reduce maximum health below 30%, you first gain a shield that absorbs 200 (+ 20% maximum health)magic damage for 5 seconds (60 second cooldown).
Unique Passive - Lifeline: If you would take magic damage that would reduce maximum health below 30%, you first gain a shield that absorbs 350 magic damage for 5 seconds (90 second cooldown). Triggering Lifeline grants Lifegrip.
Unique Passive - Lifegrip: Grants +20 attack damage, +10% spell vamp and +10% life steal until out-of-combat.