Let's look at what going in the bass clef. You are playing a B for beats 1 and 2 and then playing another B on beat 3, but you also play a D for beats 2 and 3 in the bass. Because you play the D on beats 2 and 3 and the B is also being played on beats 1 and 2, the rest is used to show you what beat to start playing the D.
Without the rest in, the notation would tell you to play the D for beats 1 and 2 instead of 2 and 3. The rest is necessary to show where to play the D. It looks odd, but it's the best way to notate what is happening.
If the lower staff was written as two separate parts in the bass they would look like this:
z D2 |
B,2 B, |