In a possible case of
transatlantic misinterpretation, the meaning of "chaser" as a
soft drink or mouthful of
beer taken to wash away the taste of a
cheap, strong alcoholic drink sounds like nonsense to my
British
ears.
The "chaser" that I know (but have not quite fallen in love with) is a small measure (usually single) of strong drink, often Scotch whisky, taken immediately after a pint of beer.
The main reason for chasers is to get drunk quickly. The large volume of carbonated liquid residing in the stomach as a result of drinking the beer is an ideal vehicle for the uptake of alcohol. But the concentration of most English beer is low - 3 or 4 percent. The addition of the 'hard liquor' ups this by at least half, leading to a greater rate of intoxication.
Of course, this method is somewhat slower than that of deriving most of the alcohol content from the hard stuff alone, since it involves ingesting several pints of liquid. However it's arguably more civilised and certainly less disgusting, since it involves drinking two things that you like (beer and whisky... or rum, or brandy, or whatever you like). If you don't like the strong drink, then you'd better forget about this idea and use mixers instead.
Even if you don't want to get drunk quickly, the chaser can be a good idea if your stomach is not very large and you've had a large volume of beer, since the liquor occupies very little space.
The downside of the British chaser is that the song Straight, No Chaser -- memorably interpreted by, among others, Miles Davis and his ensemble in Milestones makes no sense.