Bag of word, whether used with unigrams or bigrams, breaks the structure of the text. Features whose sole existence is informative regarding the concept are valuable in this context.
If you use bigrams, then the feature "White house" is a good indication to politics.
When you use unigrams then both "white" and "house" are less informative.
Note that you lose the context when using bag of word.
With respect to "white" and "house" you will get a similar representation for:
- The white house announced a new welfare policy.
- The white car parked near the house.
On the other hand, the more token you aggregate, the higher the number on ngrams you will have. The number of occurrences of each ngram will be lower so your computation will become more sensitive to noise.
Luckily, you don't have to choose either unigrams or bigrams.
A straight forward way is to use both. That will lead to many features, which is a problem on its own.
A better approaches is to use only the ngrams that are not accidental. You can identify ngrams whose occurrences is significantly higher than the expected if they were indented, lead to more mutual information or whatever measure that fits you needs. That will remove most bigrams, keep bigrams like "white house" and usually a good source for insight in manual observation.