It's not always necessary. It's down to motherboard design.
Oversimplifying somewhat, CPUs have evolved from accessing memory in 16-bit wide chunks to 64-bit wide chunks.
Memory has evolved from 30-pin SIMMs (16 bits wide) & 72-pin SIMMs to 168-pin DIMMs (64 bits wide).
The evolution of memory has lagged the evolution of CPUs very slightly (in terms of price and availability), so at one point there would have been, say, Pentium motherboards (64 bits) that had 72-pin SIMMs (32 bits). To get the 64 bits, you needed two identical SIMMs so the CPU could pull 32 bits from one, and 32 bits from the other.
There are other reasons. For example, some motherboards used "dual channel" designs to provide a 128-bit memory path, so you have to use paired DIMMs in them, too.
(SIMM & DIMM are ways of building memory sticks.)
(I've actually oversimplified somewhat: it's ultimately the memory controller that determines the memory slot design. Right now, there are memory controllers that only address single slots, only address double slots, and those that can work out whether you're using matched pairs so choose the optimum solution automagically.)