Mini glass bottles are used in many ways today. During long flights or train travel it is a lot more convenient to have a small shot designed for one person. Many hotel rooms have mini-bars and these are a great place to store several miniatures of different brands compared to one large bottle.
Hotels work with a lot customers and each of them has unique taste in things. This way a person can choose what liquor they want to drink. Large and small distilleries can use small miniatures to experiment with a new line of alcoholic brands or for promotional purposes, giving out samples of their product. For some bars it is a standard procedure to use miniatures for mixed drinks as it is a standardised 50 ml spirit bottle, the equivalent of one shot (some European countries have this set at 25-30 ml, so the mixed drink can come out stronger than usual).
Finally, liquor stores use miniatures for customers who would rather take one or two shots compared to buying a whole bottle. Often you can find several brands of alcohol packed in miniature sets together. One example would be three miniature bottles where one is a vodka mini, one would be brandy mini and the last one could be a rum mini bottle. These packages can be an excellent gift to someone who just reached legal age of drinking and can now try and taste what liquor suits them best.