One of the striking things about Coptic is how much Greek it contains. Estimates vary by text and by genre, but a Coptic biblical or monastic work can carry a fifth of its vocabulary from Greek, borrowed complete with Greek inflectional endings in some cases and fully naturalised in others. Beta sits at the meeting point of the two layers, because it writes a sound that both languages had, so words spelled with beta may be Egyptian, Greek, or Greek so thoroughly domesticated that the distinction has stopped mattering. That mixture was not a sign of weakness.
The translators who created Coptic Christian literature were working from Greek scripture, Greek liturgy and Greek theology, and they took over the vocabulary of that material rather than coining Egyptian substitutes for terms that already had settled meanings. The result is a language that can carry Greek doctrinal precision inside Egyptian grammar. Beta is also the letter around which the clearest sound change in the history of Coptic reading can be observed.
The Bohairic pronunciation used in Coptic Orthodox services today was itself reformed in the nineteenth century, when a movement to bring Coptic liturgical reading closer to modern Greek pronunciation was promoted in Egypt, and the older reading survives in some communities and in the accounts of earlier travellers. The change is a matter of record rather than of dispute, and it means that how beta sounds in a Cairo church now is not automatically how it sounded there three hundred years ago.