A profile has to be continuous while you design. You cannot design what you cannot see, and the grading profile is where the vertical elements of a road or a graded site live: the crests, the sags, the existing ground running against the finished grade from the first station to the last. You need all of it in front of you at once, not a segment at a time. The same goes for anything that runs along the profile. On a pipe network or any utility there may be a drop right where two segments would meet, and on a split profile you might miss it.
Why it still has to be split
As the design settles, the profile has to be split anyway, because that is how it goes on a sheet. Each run carries its own bands, start and end, and its own station range, and that is what makes a seam readable. One long profile cut only by match lines loses data at every cut: whatever sits under the match line falls out of the set. So each split should reach slightly before and slightly after the match line, an overlap, so a reviewer can see the seam and check it.
That is the whole argument. Continuous while you design, split for the set.
What the split costs by hand
Here is what the drafter actually does. They read how much the plan view covers, then do the math on the viewport: how much fits horizontally at, say, 40 scale for the plan and 40 scale for the profile, and how much fits vertically. The plan usually covers more stations than the profile can show, because a plan path meanders and a profile is straight, so the profile that fits the viewport is shorter than the plan above it. They work out how long the profile can be, find that it does not match the plan, and go back. It is manual, it is heavy, and it is back and forth.
Then the design changes, which it does. Even with Civil 3D's native view frames, editing after the fact is cumbersome, so what you end up with is asymmetric sheets: one with a very long profile, the next with a short stub, because that is whatever fit that day. Nobody goes back to fix it. It took too long the first time, and there is never time the second time.
And that is only the horizontal cut. Civil 3D can split a profile view on the datum, but it is a setting you work out per view: a user-specified height, where to cut, where each datum goes, and the bands on each piece. View frames cut by station and leave the height to you. Breaking a profile vertically is extremely annoying, to say the least. Count about ten minutes per view just to work out the bands and the elevation heights before you cut anything.
What you do in SheetAgent
You give it the sheet names. You give it a coverage if you want a specific one; if you don't, SheetAgent offers options at build, a 50/50 split of the sheet between plan and profile, or the alignment's own coverage. You set the plan scale and the profile scale. That is the input.
At preview, SheetAgent checks whether the plan and the profile fit the viewport at those scales. If they do not, it gives you two answers and lets you choose: move to the next scale that fits both as they are, or keep your scale and split the profile.
Choose the split, and SheetAgent opens the drawing where the profile lives, or works in the current one if it lives there. Either way you do one thing: pick where the split profiles should land in model space. You review what it proposes, move a seam if you want, and accept. From there SheetAgent runs the whole round trip: the split views are created, the drawing is saved, you are back in the layout, and the sheets are built with every profile viewport pointed at its own piece.
The continuous profile itself is never touched. The split is made next to it, at the location you picked, as its own set of views, and the profile you designed on stays whole. You can still annotate the continuous profile after the split. If you added labels post split, no worries, it happens; that is what the Sync Profile Labels button is for. Press it and SheetAgent adds those labels to the split views, so you do not repeat the work.
That is why it is faster than doing it natively, by a wide margin. The part I care about more is that the profile and the plan stay synced from then on. Move a plan tile and the split follows at the next update. And the learning curve is the paragraph above: pick a point, review, accept. Someone with very little training can do it on their own, and they will never do math on a viewport again.
The steps, from the Scale Check offer to the finished sheets, are in the documentation: splitting a profile view vertically, and splitting a long profile view per sheet.