How does "proffer" differ from "offer"?

In 2005, I asked Merriam-Webster the following question. Neil S. Serven, of the editorial department, replied as follows.

Q: Proffer means “to present for acceptance: tender, offer.” Offer means “to present for acceptance or rejection: tender,” as in “was offered a job.” What's the difference?

A: The slight discrepancy in their usage is explained in the following excerpt from the article at “offer” that appears in Merriam-Webster’s Dictionary of Synonyms:

“Offer,” “proffer,” “tender,” “present,” “prefer” can all mean to lay, set, or put something before another for acceptance. “Offer,” the most common of these words, frequently implies a putting before one something which may be accepted or rejected:

• There was a crown offered him: and being offered him, he put it by.

• Had he succeeded, he told me, he would have offered me the post of subeditor.

• Offer a suggestion.

• The dress department offers several new models this week.

• He offered $10,000 for the house.

• We must ask in the end what they have to offer in place of what they denounce.

“Proffer” differs from “offer” chiefly in more consistently implying a putting or setting before one something that one is at liberty to accept or reject and in usually stressing voluntariness, spontaneity, or courtesy on the part of the agent:

• Proffered his arm to a lady having difficulty crossing the street.

• Felt that it would be indelicate just then to ask for any information which Casaubon did not proffer.

• Rejected the proffered assistance of a couple of officious friends.

• The flavor of social success is delicious, though it is scorned by those to whose lips the cup has not been proffered.

In general, “proffer” emphasizes the active will and deliberation on the part of the person behind it, while “offer” is used in more casual contexts. A mechanic who offers to check your car’s oil level would probably not be “proffering” this service, since presumably he offers it to everyone as part of his job. A person who pulls over to help a stranded motorist, on the other hand, might be said to be "proffering" assistance, since it is being done out of courtesy, not obligation.