Substitution involves replacing variables with numbers to arrive at a full equation! Now that we all know what we are talking about we will look at real world examples!
Every employee in the US has to fill out a W-4 upon being hired in order to figure out how much tax should be withheld from paychecks. You read the statements and decide what number should be substituted for each variable (A-G) . Then you add A-G together to get the final answer of H which serves as a full equation. Many US tax forms have a similar format which requires you to follow prescribed equations with your own numbers being substituted in. If someone did not have the skill of substitution, they would have trouble filling out this paperwork and may have either too little or too much tax withheld from their paycheck. They may have to rely heavily on their boss or human resources department to help them with this personal paperwork.
If you want to be in the medical field that generally involves medicines whether you are selling, ordering, mixing, or administering them. There are strict formulas from the pharmaceutical companies and from the FDA about how much medicine to give someone of a certain weight or age and over what time period. There are also medicines that come in specific concentrations that need to be considered to get the correct dosage to the patient. If anyone along the line from the doctor, pharmacist, nurse, or manufacturer does a calculation incorrectly becasue they did not understand substitution the patient may either get too little of a dose and not reap the benefits of the medication at all or get far too much medicine and have an adverse reaction from accidental overdose. This skill is literally life and death in these situations.
Past scientists have figured out how to model vast experiences in the universe and have created equations that do just that. If you work in any engineering or science filed you will be relying on this past work to continue your own work. If you don't understand how to substitute the proper numerical conditions or constants into scientific equations you are doomed to have to derive those equations yourself which is "reinventing the wheel" rather than pushing the scientific frontier or using their equations as tools to build worthwhile projects like buildings and bridges.