I have two questions.
Marley’s ghost
It is important to remember that Jacob Marley was dead. Did Scrooge know that? Of course he did.
Scrooge and Marley had been partners in London for many years, and excellent men of business they were, too.

Q. I have no idea why there is "they were" at the end of the sentence."// If "they were" is gotten rid of ,I think, the sentence will be a perfect sentence. I think the sentence have useless words(="they were" ) now.

Q. What do "they" mean? Scrooge and Marley?

Question

I have two questions.
Marley’s ghost
It is important to remember that Jacob Marley was dead. Did Scrooge know that? Of course he did.
Scrooge and Marley had been partners in London for many years, and excellent men of business they were, too.

Q. I have no idea why  there is "they were" at the end of the sentence."// If "they were" is gotten rid of ,I think, the sentence will be a perfect sentence. I think the sentence have useless words(="they were" ) now.
 
Q. What do "they"  mean?  Scrooge and Marley?

Answer

Yes, 'they' refers to Scrooge and Marley. The sentence 'and excellent men of business they were too' is either a direct quote or an attempt to match the (now rather dated) style of Dicken's time. It is by no means bad usage as such, just dated usage. A more modern phrasing would be:

'They were known for their excellence in business'

Although there are plenty of other ways you could phrase it.