To confirm the timeframe

Jesus had just listed detailed prophecies: the destruction of the temple, persecution, tribulation, false prophets, the gospel preached to all nations, and His coming in judgment. Then He said, this generation will not pass away meaning the people He was talking to would live to see it.

To underline certainty

By saying Heaven and earth will pass away, but My words will not pass away, He was doubling down. Even if the universe itself collapses, His prophecy must be fulfilled exactly when He said within that generation. It is a covenantal guarantee, not a suggestion.

To silence doubt

Many try to say this generation means something else (like 'race' or 'future generation'), but Jesus removes that excuse. He compares the temporary nature of heaven and earth to the permanent truthfulness of His words.

Fulfilled View Summary:

This generation always means the people alive at the time of speaking just like it does throughout the Gospels (Matthew 11:16, 12:39, 23:36, etc.).

Jesus did not just say His return would be soon He anchored it to the living audience.

Matthew 24:35 is not a shift away from timing it is the divine seal confirming He meant what He said.

If His words in verse 34 failed, then verse 35 would be meaningless. But since He did come in judgment in 70 AD, His words did not pass away they were fulfilled exactly as promised.