el pulque lo sirven en las pulquerias

Hola Như Ý

Sometimes the object is repeated with the pronouns lo, la, los, las. This happens when the you start the sentence with the object: El pulque... 

Other sentences could be for example:

If you translate all these literally including the lo, las... it wouldn't sound right: 

The boxes they are putting them in the store-room. (??)

The pulque they serve it in the pulque bars. (??)

So you will normally find a different translation; sometimes expressed with a passive: the pulque is served in pulque bars

I hope this clarifies it.

Saludos