I use terracotta spikes to water some of my houseplants; are these like what you are referring to?
They are about 5"'or so in length.
In the garden I would use them to add some supplemental water to those plants that need a consistent moist soil. I have some plastic spikes that I use outdoors for this purpose but find the water bottles empty guickly and they need to be refilled often. I use 1-2 litre pop bottles; 33 oz would equal about a litre. I also have found that the 2 litre bottles esp tend to collapse, so I have cut off the bottom(top when in the spike) and then you can just pour in more water without having to remove from the spike. These work well for fertilizing your plants as well. The terracotta spikes get gunked up with soil,etc anyway so on occasion you would probable need to pull them out to clean/ soak. Roots also tend to grow in and around them as well. Hope this helps!
Btw, not sure if you're a hockey fan but your Nashville Predators are here in my city to play our Winnipeg Jets tonight and Thursday so am looking forward to some exciting playoff hockey! We love our Jets, but best of luck to your Preds; hope it's a great game.