Tradition comes from the Latin word "trahere" which means 'to hand over'. Tradition in a strict sense is the passing of something to another, and by extension becomes the word we use to describe the passing of objects, practices and knowledge to the present from past generations.
In the productive arts tradition's role is to pass on the practices of that art. In furniture making, for instance, the ways in which joints are constructed, the ways to shape and mold wood, and how to work with the grain of the wood to produce a satisfying result, are all practices that are passed from one generation of craftsman to the next. A great deal of this art may however have been transferred to books, but still the transference of knowledge from the past to the present is active.
In science, often thought the realm of radical ideas, tradition is in play even more so. Take for example physics, the physicist relies on what he's learned from books, these books written by generations of scientists before him, each generation building a theory of the cosmos upon the ones coming before him. Each generation may make modifications to the theories, but each still bases its knowledge upon things proven in the past. To throw out the traditions of the science would be to assume nothing has been proven and would demand that all assumed premises be proven again. Now no scientist is about to throw out generations of science to start a new science whole cloth, rather they stand on the shoulders of their predecessors. Science, rather than being contrary to tradition, is fundamentally reliant on tradition.
In science things are proven by argument and observation, in art through practice. Traditions pass not just what "has always been done" but rather "what has always worked." But in compounded arts, such as architecture, the aesthetic and the practical are closely related, ie roofs take the shape they do because they shed water, eaves overhang to keep rain away from foundations, and windows are the size they are to let in enough light, etc. However many aesthetic considerations are extrinsic to good building practice, and can take any number of forms, thus the wide variation in traditional styles.
But radicals in the art of architecture, who despise the term tradition, falsely conflate traditional building practice in architecture with the aesthetic traditions, and throw out the architectural baby with the bathwater. They falsely think that eaves, overhangs and sloped roofs are the self-same as aesthetic traditions and throw them all out to create a new architecture ex nihlo. The problem is, this often leads to failure, look at the example of the East Building of the National Gallery, where the traditional methods of stone cladding were abandoned for a new method, which is now a spectacular failure.
The entire facade of the East Building has had to be repaired.
Like a doctor who abandons established practice, letting an infected appendix burst to see if it would cure the patient, the modern architect would let buildings fail so that he could create something whole cloth new, but not realizing that traditions are the key component to his success.