In older toasters, the reason you had to put the bread on a specific side even when toasting a single slice is due to the way the heating elements were designed and positioned in the toaster.
Older toasters typically had two heating elements, one on each side of the toaster. These elements were essentially wires that would get hot when electricity passed through them. The purpose of having two elements was to accommodate two slices of bread, one on each side, allowing you to toast two slices simultaneously.
When you wanted to toast only one slice of bread, you were required to put it on a specific side because the heating elements were not designed to work independently. Both elements would turn on when you pushed down the toaster's lever, regardless of whether one or two slices were inserted.
If you placed the bread in the middle or on the wrong side, the heating element on that side would still heat up, but the other side's element might not come into close contact with the bread, resulting in uneven toasting. To ensure even toasting, you needed to place the single slice on the side where the heating element was more likely to come into direct contact with it.
Modern toasters, on the other hand, have evolved to address this issue. Many of them now come with a "single slice" setting, where only one side's heating element will activate when toasting a single slice of bread, ensuring even toasting regardless of where you place the slice.