Some missense variants have significant impact on the protein function, some do not. In the absence of global comprehensive functional assays of missense variants, the next best way to assess if a missense variant is likely to be pathogenic is through prediction tools which take into account factors like the chemical properties of amino acids, functional protein domains and protein conservation to predict how likely it is that a missense variant will impact function. A number of different missense pathogenicity predictors are available for human through Ensembl VEP, and these are are optimised for different purposes.
If a variant hits a splice site, you want to know if splicing is going to occur as normal, or if you can expect a different protein isoform. We have a few cool tools with the VEP that will help you to assess that for your own variants.