For example, it is "Senatus Populusque Romanus" but it could be "Senatus et Populus Romanus".
Similarly, it is "qui ex Patre Filioque procedit" but it could be "qui ex Patre et Filio procedit"
Perhaps more drastically, in the Tantum Ergo we read:
Genitori, Genitoque Laus et iubilatio, Salus, honor, virtus quoque Sit et benedictio: Procedenti ab utroque
Basically, one line uses "-que" and the next one uses "et".
Is there any fundamental reason why one method is preferred than another? The Wiktionary entry for "-que" states that:
In archaic and official language, -que is preferred to et, from which it is distinguished by denoting a closer connection.
Is that what explains the above examples? That might be clear in the case of "SPQR" and the Nicean Credo (where the filioque is sometimes found). But it is not so evident to me in the case of the Tantum Ergo. Maybe there is more to it? Is there a "canonical" answer to this issue, or is in the end essentially arbitrary, perhaps based on taste, rhymes, etc?