The reason is that someone's motive might provide them an incentive to argue for or against a particular viewpoint independent of any objective reasons for doing so. This is important because when people have a viewpoint they want to be correct, they tend to emphasize evidence or reasoning that supports their reasoning, and downplay any that doesn't.
For this reason, it is important to consider someone's motive, because you want to determine if they are adopting a certain line of reasoning because it supports the argument they want to be correct, or because they honestly and objectively believe it to be correct.
I'll give you another example: if a drug company offers you reports saying that their drug is better than the standard of care, you need to factor in their reasons for doing this. Generally, they want to make money off the drug, so they will often pick study designs, outcome measurements and so forth that are more likely to report a positive finding, and they also might not emphasize data on safety risks or other studies that don't support their drug. That doesn't necessarily mean the drug isn't any good- it could be a wonder drug- but you need to very carefully consider any evidence that the drug company offers, where it came from, and what the details say.
On the other hand, if a doctor with no financial or personal interest in the drug at hand is trying to determine the best possible treatment for his patient, goes through the evidence and concludes that a certain drug is better, then you might be more inclined to believe that, because he has no reason to support one drug over the other- the only thing that interests him is which drug the evidence tells him is actually better. So even if he ultimately uses the same reasoning or evidence as the drug company, it's more likely that he actually believes that.
Basically, most people are not totally impartial in their decisions and will often make decisions for reasons other than objective reasoning, even if they believe that their reasoning really is objective. As such, someone's motive for taking a particular viewpoint needs to be considered, as well as the extent to which that motive influences them and why.