Collecting things is one of the most popular hobbies worldwide, so it shouldn’t strike us as a surprise that coin collecting is very widespread and attractive to many people. Of course, the primary function of coins has always been that of a currency, but whether certain coins are ancient or recent, in or out of circulation, common or rare coins they are generally sought after by dedicated coin buyers and collectors. (Well, certainly there are exceptions to that, and not all coins are of the same value–pennies, for example, might not be in as high a demand as a rare ancient Roman coin.)
Coin collectors, just like most other collectors, have their own preferences as to what they consider to be valuable, or can have collections with particular themes of their interest. Many also communicate, cooperate and form communities based on their shared interest in coin collecting, not surprising since most people tend to like the company of others who share their hobbies. Coin collectors can also trade among themselves, or help their fellow buyers in finding the coins of their heart’s desire.
A great example of solidarity between collectors, in this case ancient coin collectors, is the Ancient Coin Collectors Guild, a nonprofit “committed to promoting the free and independent collecting of coins from antiquity.” There are also a great many other resources for both current and prospective coin collectors, including for instance this comprehensive list of links to quality ancient coin dealers.