你的句子没有上下文不太好理解，我特地在网上查了一下，给你补齐其上下文：

He must have been completely lost in something he was reading because I had to tap on the windshield to get his attention. “Is your cab available?” I asked when he finally looked up at me. He nodded, then said apologetically as I settled into the back seat, “I’m sorry, but I was reading a letter.” He sounded as if he had a cold or something. “I’m in no hurry,” I told him. “Go ahead and finish your letter.” He shook his head. “I’ve read it several times already. I guess I almost know it by heart.” “Letters from home always mean a lot,” I said. “At least they do with me because I’m on the road so much.” Then, estimating that he was 60 or 70 years old, I guessed: “From a child or maybe a grandchild?” “This isn’t family,” he replied. “Although,” he went on, “come to think of it”, it might just as well have been family…

由此可以看出，这个 At least they do with me because I’m on the road so much 是承上文 Letters from home always mean a lot 说的。现在我们单独来看看这句话，这个 do 的意思就很清楚了，它是代替动词 mean 的：

“Letters from home always mean a lot,” I said. “At least they do with me because I’m on the road so much.”——句意：“家里的来信总是很重要，”我说，“至少对我是如此，因为我经常在外面跑。”