Merriam-Webster Collegiate Dictionary meaning for Standard:
Main Entry: stan·dard
Etymology: Middle English, from Old French estandard rallying point, standard, of Germanic origin; akin to Old English standan to stand and to Old English ord point -- more at ODD
Date: 12th century
a conspicuous object (as a banner) formerly carried at the top of a pole and used to mark a rallying point especially in battle or to serve as an emblem
a long narrow tapering flag that is personal to an individual or corporation and bears heraldic devices b : the personal flag of the head of a state or of a member of a royal family c : an organization flag carried by a mounted or motorized military unit d : banner
something established by authority, custom, or general consent as a model or example : CRITERIA
something set up and established by authority as a rule for the measure of quantity, weight, extent, value, or quality
the fineness and legally fixed weight of the metal used in coins b : the basis of value in a monetary system
a structure built for or serving as a base or support
a shrub or herb grown with an erect main stem so that it forms or resembles a tree b : a fruit tree grafted on a stock that does not induce dwarfing
the large odd upper petal of a papilionaceous flower (as the pea) b : one of the three inner usually erect and incurved petals of an iris
a musical composition (as a song) that has become a part of the standard repertoire
- stan·dard·less adjective
synonyms STANDARD, CRITERIA, GAUGE, YARDSTICK, TOUCHSTONE mean a means of determining what a thing should be. STANDARD applies to any definite rule, principle, or measure established by authority (standards of behavior). CRITERIA may apply to anything used as a test of quality whether formulated as a rule or principle or not (questioned the critic's criteria for excellence). GAUGE applies to a means of testing a particular dimension (as thickness, depth, diameter) or figuratively a particular quality or aspect (polls as a gauge of voter dissatisfaction). YARDSTICK is an informal substitute for CRITERIA that suggests quantity more often than quality (housing construction as a yardstick of economic growth). TOUCHSTONE suggests a simple test of the authenticity or value of something intangible (fine service is one touchstone of a first-class restaurant).
Well... there is more then enough information in the above meaning to give you some idea what Standard means.
What I will focus on is item #3.
So you ask "Why have a Breed Standard?"
The purpose of such a Standard is to maintain integrity and type of the breed. It is a written example of what all breeders are aiming for when making certain choices in their breeding programs. This will in turn ensures that you as the prospective owner of a Weimaraner will be assured you have a Weimaraner not some other type of dog. The Breed Standard describes what the living animal should look like, there can be no mistakes about it. Without it, how could anyone possibly know what a Weimaraner looks like. In fact, without Breed Standards, there would be no pure bred dogs.
Included on this site; the American Standard and the Australian Standard (click on the links below to view them).