Different orchestras are seated differently. Why?
Orchestras originally had the violins on opposing sides (which is the seating that OCO uses). When the first recordings were made, recording engineers found it much easier to balance the sound when are high pitch instruments (violins) were on the same side and the low instruments (cello & Bass) on the other side. That seating became the standard modern orchestra seating.
This is a question of both practicality and "period authenticity." While there has been a "standard" western seating for almost a century, many conductors have experimented with other setups; most of these concern the arrangement of the bowed string instruments. The modern symphony orchestra grew from smaller chamber consorts, and one can view this organization based on timbre hierarchies. While the tessitura considerations, as mentioned in the other post, emerged as a recording phenomenon, this model has held for aesthetic and practical reasons. This "high-to-low" organization is, however, absent in the organization of non-stringed instruments, and conductors typically have more choices depending on the size of the hall and orchestra (e.g. whether to put the brass in a single arc). Piano, harp and percussion placement is probably the most flexible, although in most cases, the percussion appears at the extreme rear of the ensemble, the harp and piano on stage right, usually between the second violins and the winds. Some conductors reconstruct seating arrangements contemporary to those of the pieces played, but this is often eschewed since most conductors have been trained to the standard seating; cueing instruments or sections becomes a little less second-nature when a conductor has to adapt to a standard s/he is not used to, and orchestral musicians rely on spatial properties when balancing their dynamics and timbres. I’m not aware of how the unions regard seating these days and whether it is required to be fixed, but beyond this, seating is essentially a conceptual and sonic decision of the conductor made for musical purposes.
