Personally, benzos+methadone is a very unique mixture. Obviously not safe, and I'm not recommending it. But there are a LOT of people that once they stop their heroin/opioid addiction/habit and start going to the Methadone Clinic for MMT, they want a new "downer" high to replace it. Strangely enough, my daily methadone dose alone does not provide a high or "feel", and the dose of benzo by itself doesn't make me particularly euphoric, but when I take a couple xanax with my morning methadone dose, it creates a synergistic effect that is like taking a different drug altogether and I have experienced extreme nodding from the combo. Obviously opioids and benzos fight to be metabolized often times by the same liver enzymes (CYP450, CYP3A4, CYP2D19, etc.) so it increases the amount of the drug in the system since it can't be metabolized at its normal rate, causing one or both drugs to have a stronger effect. Doctors often times refer to this as "rather than 1+1 equaling 2, by mixing benzo (1) and opioid (1), you end up with something like 1+1=3 or 1+1=4.
Personally, I like to take cimetidine 200mg tabs OTC with my benzos, as long as they're ones that are boosted by cimetidine, and it's very noticeable. I even used cimetidine to boost my methadone concentrations one day and it was such a boost that I ended up vomiting because it was so strong in my system apparently. Also, a bit of advice: cimetidine (Tagamet, Tagamet-HB) is expensive as HELL in drugstores like CVS, walgreens, Rite Aid, etc, including their store-brand generic Cimetidine 200mg tablet bottles. At my local CVS, their store branded cimetidine 200mg bottle of pills is like 60 tabs for about $12-15. However, at Walmart, a bottle of Cimetidine 200mg x 60 tablets is just under $5, a HUGE savings, and so far the only place I've found with them at this low of a price.
To answer the OP's thread title question directly: basically it's simple, a lot of opioid users love their downer and adding in another strong depressant (benzos) creates a very elevated high due to the high of the opioid, the high of the benzo, added to the potentiation factor, and you get a super high for the user. Whereas with cimetidine, it alone has no psychoactive effects so it would only add length to the opioid high or make it more intense by a bit.