Nothing to do with speed control.
There will be occasions, due to airspace, when you can't descend with the ILS. With this in mind the instruction is split - first, get you on the localiser, then clear you to "descend with the ILS" when other airspace / procedures allow you to.
To give an example. You're cleared through another airfields airspace at FL50, you're already localiser established. As the glideslope needle goes through the centre of the HSI you have to maintain FL50 until given further descent. But now you're above the glide - so, ATC will, when they can, descend you to a level which will get you back into a position where you can intercept from below. You should not follow that glideslope needle until the instruction "descend with the ILS" is received