Say you are going down the field from the 1 yard line. If the receiver catches the ball and his feet are at the 10 but the ball is extended behind him at the 8. Is the ball marked down at the 10 or the 8 yard line.
The ball is the determining factor in the spot of the next down. This is discussed in a few places in the rules, but the most clear spot is Rule 7, Article 3:
ARTICLE 3. FORWARD PART OF BALL
The forward part of the ball in its position when it is declared dead in the field of play shall be the determining point in measuring any distance gained or lost. The ball shall be rotated so that its long axis is parallel to the sidelines before measuring, while maintaining the forward most point.
This is why receivers will often extend the ball in front of them when their momentum is taking them out of bounds but there is no significant risk to ball security, or when they are close to the line to gain in order to gain a first down or touchdown.