A moss pole gives a climbing houseplant the chance to do what it does in the wild: grip a damp vertical surface with its aerial roots and haul itself upward. For aroids such as monstera, pothos and philodendron it often means bigger leaves and a stronger plant.
Why it helps
These plants are climbers by nature. Given a damp, textured support, their aerial roots attach and draw up water, and the plant frequently responds by producing larger, more mature foliage than it would scrambling along the ground or flopping out of a pot. In many aroids the leaves grow bigger and more deeply cut the higher the plant climbs, and a pole is what lets that happen indoors. It also keeps a big plant upright and tidy instead of sprawling across a shelf.
Sphagnum or coir
This is the one moss-wall context where sphagnum is exactly right. A pole stuffed with damp sphagnum holds water and stays moist, which is what the roots want, and being upright and regularly watered it does not suffer the way sphagnum would on a dry vertical wall. Coir poles are tidier and longer-lasting but hold less water; sphagnum is messier but the roots love it. Whichever you choose, the point is a surface that stays reliably damp for the roots to grow into.
Making and using one
Form a tube of plastic mesh, pack it firmly with pre-soaked sphagnum, and stand it in the pot behind the plant, anchored into the soil so it cannot rock. Tie the stems in loosely to start, guiding the nodes and their emerging aerial roots against the moss. Then keep the pole damp: mist it daily or pour water down the top, and the roots will grow into it and grip on their own within a few weeks. Extend the pole as the plant climbs.
Keeping it damp without the bother
The whole benefit rests on the pole staying moist, and a dry pole is the usual reason aerial roots refuse to attach. Daily misting works but is easily forgotten; growers with several plants tend to rig something steadier. A length of capillary wick run from a small reservoir at the top, a self-watering pole with a hollow core you simply top up, or a weekly thorough soak of the whole pole in the shower all keep the sphagnum damp with less fuss than hand-misting. Warm, humid air helps too, which is why these plants and their poles do well in a steamy bathroom.
Extending and the usual mistakes
Add to the pole before the plant overtops it rather than after, since a stem that has grown past its support flops and is awkward to retrain. Extending is easiest with a stackable or telescoping pole, or by lashing a second mesh tube onto the first and packing it with fresh damp sphagnum so the roots carry straight on up. Most problems come from two habits: letting the pole dry out, so the roots never grip, and tying stems so tightly that they are throttled or snapped. Tie loosely with soft ties, guide rather than force, and let the plant do the climbing itself. A pole that stays damp and is extended in good time turns a sprawling, leggy aroid into an upright specimen with markedly larger leaves, which is the whole reason to bother with one.