If you are talking about 7-mode, you should review the HA guide. IT's for uptime and in 7-mode it takes the personality of the other one
Simple concept. Two storage controllers working as a HA Pair, each able to "take over" its partner's workload during a planned or unplanned outage, which it can later "give back" when the partner is ready.
Please go through the KB below which explains what exactly happen during take over and give back and also please go to HA guide Page:- 20 to understand HA Pair works ?