The individual sectors allow you to do the weightings yourself and adjust for risk and preference versus a total market fund.
I have an asset allocation plan that includes some sub-segments of the total market. I personally would not choose to hold every sector separately.
Within my asset allocation plan. I do hold some ETFs in a couple of sectors in addition to more broad Large Cap (S&P 500) and Small Cap funds,.
I have ETFs in Health Care RYH (Large Cap) / PSCH (Small Cap) and Consumer Staples - RHS (Large Cap) / PSCC (Small Cap), which are defensive sectors that did relatively well in prior downturns.
These are permanent allocations. I do not try to rotate among sectors. I only re-balance as they go up (trim) or go down (add).
I have a lot of Mid/Small-cap as well as Value bias and hence my sector weighting is different than total market. I have overweight REIT (on purpose) thru VNQ/VNQI. I was underweight on Tech and recently bought IGM to compensate that. I'm still underweight Healthcare - will need to do something about it.
4 out of 5 funds I hold are sector funds which are managed MF's......I do not like ETF's at all.
Total Market S&P in mutual funds, not ETFs.