At the very beginning of my answer, I would like to remind that everyone should be familiar with the type of charts he is using. A thorough description (PDF) of all charts published by the FAA is available on this FAA website. Refer to page 77 of the PDF document to find the altitudes your question is dealing with. On every flight, you make decisions based on the information you read from those charts.
Basically, once you are cleared for an ILS approach and have intercepted the glidepath in accordance with your clearance, your task is to track the localizer and glideslope, allowing not more than a half-scale deflection, until reaching your DA (or DH on CAT II and III approaches). The exact altitude reading at which the glideslope passes over a specific point is as variable as air density is. This might cause you to be more than 20 feet below the minimum altitudes, but it doesn't matter as all of them past the FAP are for non-precision approaches only.
I don't know how it's handled in the US, also the FAA is using the term FAF - Final Approach Fix - talking about ILS approaches. Technically this is wrong, because the point where you hit the glidepath at a certain altitude is variable - remember the air density. That's why we should talk about a FAP - Final Approach Point.
Anyway, those altitudes should be used when shooting an ILS approach in order to make sure that you didn't fly a side lobe of the glideslope - even if the chances to find one are very remote. On Jeppesen charts you find this 'check altitudes' above the depicted glidepath often at the outer marker (if available) preceeded by the letters GS. On FAA charts these are referred to as recommended altitudes. Find them for example on the chart for the ILS runway 28L approach attached to the question. They are also printed above the glideslope but not underlined, as they are recommended but not mandatory / minimums.
On an ILS approach where you are tracking the localizer, but not the glideslope, because you are too far out and below it, or do not receive it. For example at HEMAN 3100' are important if ATC already cleared you for the approach and you like to descend below the last cleared (maintain xxx feet until established on...) altitude. The latest point where you have to intercept the glide slope is the FAP (FAF according to FAA) but if it is not disagreeing with your clearance you can of course get on glideslope earlier. In this case, prior to the FAP, this minimum altitude is then still mandatory for you (to be at or above) because of reduced glideslope precision at higher distances (it's a cone not a string), but I've never seen someone getting below those minima flying the glide path. In most cases the glideslope will take you quite above these altitudes and you have a continuous and more economic descent into your runway, rather than doing a stepped descent allways being at the minima.
For those who like to take a closer look at Jeppesen's charts, visit their website to find the chart legend (PDF) - take a look at page 55 of this document.