Or HaChaim on Numbers 11:16/אור החיים על במדבר י״א:ט״ז

1 אספה לי שבעים איש, "gather for Me seventy men, etc." G'd did not say: "gather the seventy elders for Me." This is proof of what Bamidbar Rabbah 15,21 stated that the elders who had accompanied Moses on the way from Egypt had all been killed at the same time as Chur the son of Miriam. This is why G'd instructed Moses to appoint others in their place from amongst the elders of Israel. G'd added the word לי "for Me," to tell Moses that he must have in mind that these elders would perform a task for G'd so that he would not be misled by Satan to appoint someone not qualified. Once Moses had the right intention in appointing them he would be protected by the principle that people in the process of performing a good deed will not be thwarted in their intention.

2Another meaning of the word לי is that G'd would confirm the appointments and the authority of these elders would be derived directly from G'd rather than merely from Moses. G'd also used this word to encourage people to accept this position as they might have been afraid remembering the fate that had befallen the first set of elders who had been murdered by the mob. By describing the appointment of these elders with the word אסיפה, a term used for death, G'd made clear that He considered the elders who had been killed as having died in the service of the Lord. Compare Psalms 44,23 כי עליך הורגנו כל היום, "for it is for Your sake that we are getting killed all day long."

3זקני העם ושוטריו, the elders of the people and its officers." They should be people who could command the respect and obedience of the people. All this is implied in the word ושוטריו.