Cacti are naturally grown plants that can usually be found in 2 biomes:
Cacti that you can discover in the game world are already fixed parts of the template Worlds that all Creativerse game worlds are based on.
Green Cacti do not grow by themselves after world-generation and cannot be "grown" by players - only Cactus Flowers can still additionally grow after game worlds have been created, and can be grown by players too.
1-2 Cactus-pieces (-blocks) and/or rarely even Cactus Flowers will also be "dropped" occasionally by Trogs, Dried Leafi or Dustevils; and/or can be harvested from these animals if you make them your Pets and feed them their favorite food.
Cacti can make Dried Leafies, ordinary green Leafies and/or agressive Dustevils spawn, rarely also Pigsies (spotted since update R22 in September 2015), even when you plant Cacti on crafted blocks. It seems possible that during ingame-nights Cacti might also make Night Leafies spawn, even though this rarely ever happens.
"Naturally" generated Cacti as part of the template worlds of Creativerse consist of 2-5 blocks of either Cactus or Cactus Flowers. Cactus Flower are useful ingredients for a number of recipes, like Corruption Resistance Potions and Lumite Mining Cells that will require Cactus Flowers to be made, but also Basic Health Potions, Stone Mining Cells, Speed Potions, Heart-Shaped Boxes and many other recipes that will accept any kind of Flowers.
You can harvest Cactus blocks as well as Cactus Flowers without needing to equip any Power Cells.
You can grow Cactus Flowers easily yourself by placing Cactus-blocks close together, preferable in the shape of pillars. You can do this on any kind of block (even crafted ones) except liquid Lava, in any kind of environment (also on the Corruption layer as well as on Snow or even Ice), and Cactus Flowers won't even need light to grow.
Still you will have to place a certain number of Cactus blocks (at least around 9 blocks or more) into an area until Cactus Flowers will randomly start to spawn here and there.
It seems that the higher you "staple" Cactus blocks, the faster Cactus Flowers will grow on top. Cactus Flowers don't often spawn directly next to other Cactus Flowers (more often diagonally), so building walls could be a waste of Cactus blocks.
However there is a limit for how many Cactus Flowers can grow within an area, so placing too many Cactus blocks close together is definitely wasteful. This also means that you will have to harvest Cactus flowers that have grown before new flowers will be able to spawn in their stead.
Warning: Cacti are flammable, so take care to not place fire or torches too close, especially in hot environments. Hardened Lava, Igneous Rock and Sulfur can burn Cactus-blocks if you place them into hot surrounding even without any open flames nearby. The environmental temperature will decide if flammable blocks like Cacti will start to burn or not.
So if you try to grow Cacti on the Lava layer (which is not really recommended anyways), you should take care to leave at least 3 blocks space upwards, as otherwise cactus-flowers that grow on top might touch hot rocks and could then set the whole cactus on fire plus every other cacti or flammable objects and materials within reach of the flames.
Cactus-blocks re not required for any crafting recipes, however Cactus Flowers are. Since they are easy and fast to grow, they can easily be used for all crafting recipes that will accept any type of Flowers, like Basic Health Potions for example.
Cacti can be displayed on Placemats, Stone Wall Shelves and Hidden Temple Altars, in Wood Planters and Flower Pots, on Galactic Grav-Chambers, Frozen Containers, Snow Buried Containers and the like. Some of these display containers/objects will shrink Cactus-blocks in size for display.
Please note that you cannot grow Cacti nor Cactus Flowers by or after moving them into the slots of Wood Planters or Flower Pots. They will simply stay preserved there for decoration.