Apart from answers in which life gain is demanded by the metagame, or when cards synergize / trigger on life gain...
Life gain can tactically combine with control strategies. For instance, Ivory Tower and Wrath of God form a tactical combo. If you hold Wrath of God for a long time and allow your opponent to accumulate a lot of creatures, then the upside is that the Wrath of God will destroy many creatures and yield perhaps 6-for-1 card advantage; but in the process of waiting for the opponent to play out all of these creatures, you will have to withstand many attack steps and may take dozens of damage. Not to mention that the opponent has minimal incentive to overextend that many creatures. On the other hand, if you cast Wrath of God very early, when the opponent's board development is small, then the opponent won't get many opportunities to attack you and you won't take much damage. The downside to blowing your Wrath of God early on in the opponent's board development is that you won't kill many creatures, and the Wrath of God may only achieve 2-for-1 card advantage. How can you achieve the best of both worlds? If you had an Ivory Tower in play, then the opponent wouldn't be able to achieve a net reduction in your life total unless they put several creatures onto the battlefield, not merely 1 or 2. Ivory Tower's life gain also allows you to hold onto Wrath of God for longer and get more card advantage out of it without dying to damage in the meantime. So recursive life gain combos with board sweepers. There also exist successful Turbo-Fog strategies, and in some respects, life gain is similar to Fog-type cards. On turn 2, a card like Heroes' Reunion might even yield a better net result than Fog. Also, any sort of recursive life gain card (i.e. if it were powerful and it had buyback) can allow you to essentially ignore most threats on the board without having to truly control it. Gaining life faster than threats can deal damage is very similar to having Moat in play and having effectively blanked the threats. Doing so also gives you the luxury of saving your control spells for only the threats that are truly dangerous.
Another reason you may want to play life gain is if a specific card has raw efficiency. For instance, suppose one card dealt 3 damage for 2 mana; and suppose another card dealt 2 damage and gained 6 life for 2 mana. Which one would be a better choice to play? In more aggressive strategies, 3 damage might be better, since it better aligns with your game plan. Even in an aggressive deck, that life gain can buy you more turns, which equates to extra draw steps, which means drawing and casting more burn spells before you die, which ultimately advances your aggressive strategy -- hence Lightning Helix may be better than Flame Rift and/or Boros Charm if you are Boros Aggro. In more controlling strategies, 2 damage + 6 life might be better, in part because it is probably better value, and partly because staying alive is among control's strategic objectives. In combo, which often is only concerned with short-term survival, 2 damage + 6 life may be even more the best choice, since life gain only aids with temporary survival anyway. Basically, my point is that if a sorcery gains N life for 1 mana, there is a number N that is so high that the card would be too efficient to pass up on.
Magic decks tend to start the game with the same starting life total and the same number of cards. If there were factors that affected this, it could reward life gain. For instance, the scarcely-played Vanguard format modifies your starting life total and starting number of cards in hand. It is also possible that there could be a format where you frequently damage yourself with your own cards, like Underground River, Gitaxian Probe, Skeletal Scrying, and Dark Confidant. If your deck has obscenely efficient forms of card drawing and inflicts lots of self-damage, then life gain cards could be justified. Ideally, the life gain would be stapled to another substantial effect, like Pharika's Cure, Corrupt, or Lone Missionary (acknowledging that this question was primarily about naked life gain). However, when you play a self-damaging deck with life gain cards, then there would be some inconsistency between the games where you do draw life gain vs when you don't -- they would play out very differently, since some games you would be drowning in card advantage while near death, and other games you would have a healthy life total but fewer cards.