If you have boys you have probably found that they have a bad habit of developing holes in the knees of their pants. I don’t know why but for some reason between the ages of 5 and 12 my boys have succeeded in wearing out the knees of most of their pants. I have bought three times as many pants at that age than any other item of clothing. I initially tried patching them but that never worked and they looked horrible.
Their pants were weak and worn from use just like in the verses discussing old garments (Matthew 9:16, Mark 2:21, Luke 5:36). Old garments are typically well used. They have served their purpose. There comes a time for them to be put away.
There are several verses that refer to the Law as weak or useless.
Romans 8:3-4 (ESV)
3 For God has done what the law, weakened by the flesh, could not do. By sending his own Son in the likeness of sinful flesh and for sin, he condemned sin in the flesh,
4 in order that the righteous requirement of the law might be fulfilled in us, who walk not according to the flesh but according to the Spirit.
Hebrews 7:18-19 (ESV)
18 For on the one hand, a former commandment is set aside because of its weakness and uselessness
19 (for the law made nothing perfect); but on the other hand, a better hope is introduced, through which we draw near to God.
Now we know the Law didn’t start out weak and useless but the Law had served its purpose. The Law worked its purpose but now there is a better law, a newer garment. The new garment is imperishable just like the garments of the Israelites in the desert.
Nehemiah 9:21 (ESV) Forty years you sustained them in the wilderness, and they lacked nothing. Their clothes did not wear out and their feet did not swell.