What the difference between getting sentenced to jail for 100 years to 250 years? Why not sentence them to life in jail?

All is dependant on the state:
life means 20 to 25 years. After 8 to 15 years you can get parole
100 years sentence means parole eligible at 25 5o 35 years.
200 years (served consecutively) means no parole for 50 to 60 years.

Many times with multiple charges they give multiple life sentences but they are served not in order, consecutively, but at the same time, concurrently.

Remember the court language and all legal stuff has nothing to do with normal English.

The difference is when the person can be released early. Typically, a person can be released after serving as little as 1/3 of a sentence, or 25 years of a life sentence. So a person sentenced to 100 years could be released as soon as 33 1/3 years, but a person sentenced to 250 years wouldn't be.

The reason not to sentence them to life is that they could get out sooner with a life sentence.

Sentences of 100 or 250 years are not actual sentences for individual charges. They are the TOTAL of shorter sentences for multiple charges. Often no single charge is eligible for a life sentence. If one or more of the convictions is overturned, the sentence(s) for the remaining charges are not effected.

150 years apart.

because the sentences were written for each type of crime

Because of parole. If you're sentenced to 100 years, you stand to get paroled quicker than if you're sentenced to 250 years.

In some jurisdictions 'life' means eligibility for parole in X number of years... a longer sentence negates that.

Some places just sentence a criminal to ' life with no possibility of parole'.