Ooops, that's three questions!
It’s late for me but I will give you the short version to your questions.
1) Why would God divide His dealings with mankind into dispensations?
It has to do with the definition of dispensations (look it up, it is economy related)
2)Why not just reveal everything all at once?
For us today in the dispensation of grace, everything has been revealed Gen-Rev KJB 1611/1769 the preserved Word of God.
3) Does one dispensation end before another begins?
Yes...although man may not recognize the end from the beginning for quite some time.
For instance Time past began in Gen 11 with Abraham.
Time past ended with the stoning of Steven.
But now... started with Paul on the road to Damascus and will end when the church the body of Christ is caught up to be with the Lord.
Time to come... will start after we are caught up, but it will take some time, I think, for Israel to be reconstituted and the tribulation to begin.
Don’t trust me as to these things...STUDY... like the bereans...wether these things be true. Romans thru Philemon are the scriptures written specifically to us and for us in this the dispensation of grace. Paul said: 2 Timothy 2:7 Consider what I say; and the Lord give thee understanding in all things.