Depending on the figure and person, it could have been a toy that someone had as a child. For nostalgia’s sake. Many people enjoy coming back to or finding a figure that they once played with or loved as a child. And that figure now brings with it a flood of joyful childhood memories.
For other people, Funko Action Figures are a work of art. Many companies now offer action figures with extremely realistic details. MacFarlane toys broke the mold when it came to action figures, average people could now spend ten dollars on a figure that looked like a professionally sculpted statue. Up until then, the largest of your action figures weren't extremely precise or articulated. Hence, some figures are collected for the purpose of maintaining a non-expensive figure based on a show, animation character, or traditional figure.
Many people, like myself, collect action figures based on comic book characters to accompany my comic book collection. It's the sense of having a real, physical link to a character that was otherwise only featured in print within the pages of a story. People now have a Spiderman or Batman in person, in a sense.
Many figures were terminated or reassembled and are remarkably rare and a possibility of finding is impressive and gratifying.
You can collect for complete series. Tons of figures come offered in a line or series with an excuse to assemble the series. A buildable reward figure. Every figure in the series appears with a piece to collect to build a completely new figure that can't be acquired outright.
Others collect as an investment or venture on the expected value of a figure based on rareness. Typically they save the figure in the original packaging and stash it away or advertise it. Some buy various figures of the identical type and open one to play with and keep one in pristine condition. In the future, the value may go up and they can earn a profit. It doesn't regularly turn out that way, but some figures in the events have grown greatly precious and now value thousands of dollars.
And then there are people that are fans and keep them around for no other purpose than they like to buy and collect stuff based on a show or movie or comic character.
And then there are those figure scalpers. Yes, it's a real thing. They know when a figure is scheduled to be released, where to find it. Rather they have contacts in the store to tip them off of a figure’s arrival. These people will miss work or school to get to The store the second it opens to be the first person to buy the new figure. They don't just buy one or as previously stated, buy two, to open one and stash the other.