As always, "ït depends" what you are trying to show/prove/learn.
Let's assume you have 30 minutes of data that is collected on 20 second intervals.
If you just calculate the maximum value, then that could be representing different situations:
For example, have a look at these 2 extremes
1) all your values are 10, and you have 1 maximum of 20 on 900 data points
2) all your 900 data points have a value of 20
To discover that these are entirely different situations, you could also include the average. And perhaps even the standard deviation.
So your 900 values become 3 values. Nice reduction of data, but at the cost of loosing detail.
Ultimately, you could decide to store all data points, and for example plot them in a curve.
That will give the best representation of what actually happened.
But don't forget that the value for the 20 seconds interval is already an aggregate or an average.
And 20 seconds is a lifetime in the IT world.
Every sampling, by definition, throws away granularity.
I know, this was not really an answer on what you should do.
But perhaps it gave you enough food for thought to consider what would be the best solution in your case and with your requirements ;-)