Most liqueurs - and almost all the old ones - are made by infusing the flavourings in the raw spirit then sweetening. Occasionally the infused spirit is redistilled to give a stronger, more concentrated tincture, and many bitters are also made this way.
If the first thing to come to mind when liqueurs are mentioned is sticky, sickly potions sipped in thimble-like quantities by maiden aunts at Christmas time, think again. Varying in strength from around 15% to over 50% abv, and in flavour from sweet fruits to astringent herbs, liqueurs are indispensable for adding flavour and interest to base spirits when making cocktails.
If the formula for a proper cocktail is spirit, sugar, bitters and water, you can argue that a single glass of triple sec fulfils the brief.
Almost every classic cocktail contains one or more liqueurs, and they have been used since the earliest days of the Punch as a way to add flavour and sweetness to a spirit base.