Luke Dashjr answered this on IRC:
The primary purpose is to definitively indicate the soft fork is active on the chain. While it still relies on enforcement, this ensures there is no dispute over what the correct rules are. As a side effect this also makes it easy for any group of dissenters to reject the new rules. The intent of a soft fork should never be to literally force new rules (upon dissenters)
(Luke is referring to dissent against the soft fork rather than dissent against the activation mechanism set in a Bitcoin protocol implementation.)
David Harding added that MUST_SIGNAL forced signaling is there to ensure the activation of the soft fork for all nodes setting LOT=false when the rest of the network is setting LOT=true.
I run a node with LOT=false; everyone else runs a node with LOT=true. At block xxxxxx, y'all start enforcing taproots rules, but I never saw any signal, so I continue treating taproot transactions as anyone-can-spend, which is bad for me personally. If there are a lot of people with LOT=false, it also makes it unclear whether taproot is really being enforced, increasing the risk that miners may try to steal funds sent to taproot outputs.
In addition David Harding argues that forced signaling is not particularly dangerous.
What makes forced signaling so dangerous? We had that with the BIP34, BIP66, and BIP65 forks. Except for a hiccup with BIP66 due to spy mining, I don't think there was any problem. BIP8 forced signaling is also only required for a brief period, so any disruption should be short