In numismatics, there is a strong tradition of set-building. Collectors often have the goal of assembling interesting, themed and relevant groups of coins such as buying one of every date, design type and mintmark. The basic concept is to establish a collecting objective and work towards completing it. Owning a complete set increases the value of coins compared to owning individual coins.
Some series are better for set-building than others. Some coinage series are filled with key dates that are difficult to find and prohibitively expensive. For instance, it is currently impossible for a collector to build a complete set of Three Dollar gold pieces because the unique 1870-S is impounded in a museum. $20 Saint-Gaudens Double Eagles are another example. There are at least 10 different dates that cost well into five, six, or even seven figures. A complete set can be assembled, but it would require a massive budget.
The $2.50 Indian is the only United States gold series that can be affordably completed.