Last season Philadelphia desperately chased after Kesler offering him a contract that was 60% larger than what the media reported the Canucks were offering him ($1.2M). Kesler likely would've signed for $1.5 million, but the stubborn Canucks refused to make any reasonable offers and ended up paying $1.9M
If you use the 60% figure as a reasonable approximation of what Buffalo was offering Vanek, it means that Buffalo was likely offering about $4.5 million to Vanek. Vanek insulted and frustrated by the offers Buffalo was giving him, signed with Edmonton at $50M over 7 years.
Compared to UFAs
Now you have to remember that offer sheets are not comparable to UFA contracts as they include additional costs: draft picks. They are likely somewhere between 90-95% of what the same player would command as a UFA.
Value of RFAs
In general RFAs sign for about 75% of their UFA value. The offers made to both Kesler and Vanek were probably closer to 50-60% of their true value. Making these offers significantly less than the 75% they were expecting. Making the offer sheets extremely attractive relative to their current situation. Basically these teams negotiated with these players in "bad faith" as a result they were left with a very large bill.
Therefor these offer sheets are important to make sure that RFAs get fair deals and aren't cheated out of what they are worth.