SATs are used as a baseline for all state secondary schools to calculate progress - a measure called 'Progress8', which is published in the DfE performance tables and is part of teh data used by ofsted to make a judgement on the chool.
The progress8 data has a 'coverage' percentage, which shows how many children in that cohort came up without SATs results - so for example I can use that to estimate that a selective school has c. 25% of pupils coming at 11 from private preps, as their progress8coverage is under 75%.
This is why schools use SATs to set GCSE targets: schools are trying to maximise their progress8 by making sure that each child makes the best possible progress from their starting point.
Having no SATs data will probably work out OK if your child is an average-ish achiever with a decent work ethic - they'll be set 'average' targets and as long as they progress along an average trajectory, all will be fine.
If, however, they are in fact a higher achiever with a poor work ethic, having no SATs can cause their underperformance to go undetected - they're doing 'averagely', their results won't count against the school too badly, not a big deal. Faced with e.g. scarce 'extra booster group' resources, or a 'who should be pushed up into the higher set / entered into the higher level exam', the children selected might be those who SHOULD be doing better vs SATs results but aren't, not your son.
i know schools SHOULDN'T be like this. They should be spending their time getting to know the nuances of exactly how your DS ticks, and disregarding all data and targets to ensure he does the best he possibly can. However,. schools are data driven places - think about whether it was a good Ofsted report, or good results, or good position in the league tables that attracted you (and the unit of funding that is your DS) to a particular secondary: that is exactly why schools are data driven - and not having the data can leave you in an 'OK' limbo.