Most importantly, ease in finding an event or venue for your guests! We were able to easily find events around our area that we could host as they had space available which was perfect since I wanted something local but not too close by (since my guest are always coming from different places).
Sometimes it can be difficult if you don't know what kind of theme/idea would suit best with their preferences when searching online - so make sure you do some research first before booking anything!! Some people have also said there is sometimes slight delays between choosing certain things like themes etc because its mostly done through email though this has never been long enough to really complain about much either way. If looking at hosting parties then having easy access via website makes life easier than using other websites where everything needs signing up into multiple accounts just to get anywhere near all options.