From the mid 60's to the mid 70's, many people were buying automatic record changers (not only between these years, but this was the most popular time). These turntables would play one side of a record and then the tonearm would return to the beginning position, another record would automatically be dropped on top of the existing, and then this record would be played. This way you could listen to multiple sides of albums without having to go to the turntable and flip a record.
Therefore, many double albums were pressed with Sides 1 and 2 on different pieces of vinyl (as well as 3 and 4, etc....). So now you could listen to two sides of a record in order without having to go to the turntable and turn it over. Then you would only need to flip the records once and you can listen to the next two sides.
This was extremely convenient in the day where vinyl was the medium of choice.
This did cause some confusion in the record industry though. When Issac Hayes released his Black Moses album in 1971, the album was pressed in the sequence stated above. When this album was transferred to CD in 1989, they put the sides in the wrong order. Side 4 was between Sides 1 and 2. Ok anybody can make a mistake. Oddly enough, when they re-released the album again in 2009 they still did not fix the mistake.
It's a great album regardless, but its nice to hear it in the order that Issac Hayes intended.