The usage of ensure vs. insure is very important from a contractual perspective, while they sound very similar they can have very different meanings in a legal context.

Insure means to safeguard against loss or damage, typically through the purchase of insurance to do so. If a contract stipulates that the individual must insure something the contract is stipulating that the individual must purchase insurance to safeguard that asset.
Ensure is very similar in that it means to make sure or guarantee something, though the specific emphasis on doing so via insurance is not there. If a contract stipulates that an individual must ensure something is taken care of there is no requirement to purchase insurance, rather the individual is to "do their best" to make sure something is taken care of.

The onus on the party varies greatly depending on the word used, as can their legal obligation. Making sure the correct use of ensure vs. insure is used in your legal documents is something you always want to be aware of. Doing so incorrectly can result in unexpected and potentially significant costs.