Increasing Mana regeneration
Meditation is a great choice for Paladins as it greatly increases their mana regeneration rate, which can help them avoid the need to stop casting and use a potency. It also stacks with other sources of mana regeneration like items, Warmth, and Mana Recharge Shrines, allowing for an extended period of time without needing to spend mana on other skills.
Aura synergy
Meditace d2 is a runeword that gives the Paladin a level 12-17 Meditation aura that regenerates 75% of their mana. This is a huge boon for low-to-mid-game characters that struggle with mana-intensive builds and few tools to deal with it. It can even come in handy for casters who rely on attacks from the Combat Skills list and need to regenerate their mana with other means.
Aura stacking
Prayer and Meditation auras from different players or mercenaries are not stacked together, but if the same level is used for both, they stack. For example, if 8 players hire a mercenary with 23 level Prayer and 15 level Meditation (from Insight runeword), 496 life is healed every 2 seconds.
Higher rolls on Insight also regenerate mana faster, so a larger mana pool can soften the blow when it's used. This makes Insight a popular hireling weapon for Mana-hungry builds, and it's often the choice of Mercenary if the player's skill is more in the area of casters than melee.